Any substance consists of particles of a certain structure (molecules or atoms). The molar mass of a simple compound is calculated using the periodic table of elements of D.I. Mendeleev. If it is necessary to find out this parameter in a complex substance, then the calculation is long, and in this case the figure is looked at in a reference book or a chemical catalog, in particular Sigma-Aldrich.
Molar mass concept
Molar mass (M) is the weight of one mole of a substance. This parameter for each atom can be found in the periodic table of elements, it is located right below the name. When calculating the mass of compounds, the figure is usually rounded to the nearest whole or tenth. For a final understanding of where this value comes from, it is necessary to understand the concept of "mole". This is the amount of substance containing the number of particles of the latter, equal to 12 g of the stable isotope of carbon (12 C). Atoms and molecules of substances vary in size over a wide range, while their number in a mole is constant, but the mass increases and, accordingly, the volume.
The concept of "molar mass" is closely related to the Avogadro number (6.02 x 10 23 mol -1). This figure denotes a constant number of units (atoms, molecules) of a substance in 1 mole.
The value of molar mass for chemistry
Chemicals enter into various reactions with each other. Usually, the equation for any chemical interaction indicates how many molecules or atoms are used. Such designations are called stoichiometric coefficients. They usually appear before the formula. Therefore, the quantitative characterization of reactions is based on the amount of substance and molar mass. They clearly reflect the interaction of atoms and molecules with each other.
Calculating molar mass
The atomic composition of any substance or mixture of components of a known structure can be viewed from the periodic table of elements. Inorganic compounds, as a rule, are written by the gross formula, that is, without designating the structure, but only the number of atoms in the molecule. Organic substances for calculating molar mass are indicated in the same way. For example, benzene (C 6 H 6).
How is molar mass calculated? The formula includes the type and number of atoms in a molecule. According to the table by D.I. Mendeleev, the molar masses of the elements are checked, and each digit is multiplied by the number of atoms in the formula.
Based on the molecular weight and type of atoms, you can calculate their number in a molecule and draw up a compound formula.
Molar mass of elements
Often, to carry out reactions, calculations in analytical chemistry, and the placement of coefficients in equations, knowledge of the molecular mass of elements is required. If the molecule contains one atom, then this value will be equal to that of the substance. In the presence of two or more elements, the molar mass is multiplied by their number.
The value of molar mass when calculating concentrations
This parameter is used to recalculate almost all methods of expressing the concentration of substances. For example, situations often arise when determining the mass fraction based on the amount of a substance in a solution. The last parameter is expressed in the unit mol / liter. To determine the desired weight, the amount of the substance is multiplied by the molar mass. The value obtained is reduced by a factor of 10.
Molar mass is used to calculate the normality of a substance. This parameter is used in analytical chemistry for performing titri- and gravimetric analysis methods when an accurate reaction is required.
Measurement of molar mass
The first historical experience was to measure the density of gases in relation to hydrogen. Further studies of colligative properties were carried out. These include, for example, osmotic pressure, determination of the boiling or freezing difference between a solution and a pure solvent. These parameters directly correlate with the number of particles of matter in the system.
Sometimes the measurement of molar mass is carried out for a substance of unknown composition. Previously, a method such as isothermal distillation was used. Its essence lies in placing a solution of a substance in a chamber saturated with solvent vapors. Under these conditions, vapor condensation occurs and the temperature of the mixture rises, reaches equilibrium and begins to decrease. The released heat of vaporization is calculated from the change in the heating and cooling rate of the solution.
The main modern method for measuring molar mass is mass spectrometry. This is the main way to identify mixtures of substances. With the help of modern devices, this process occurs automatically, only initially it is necessary to select the conditions for the separation of compounds in the sample. The mass spectrometry method is based on the ionization of a substance. As a result, various charged fragments of the compound are formed. The mass spectrum indicates the ratio of the mass to the charge of the ions.
Determination of molar mass for gases
The molar mass of any gas or vapor is easy to measure. It is enough to use control. One and the same volume of a gaseous substance is equal in quantity to another substance at the same temperature. A known method for measuring the volume of steam is to determine the amount of displaced air. This process is carried out using a lateral arm leading to the measuring device.
Practical use of molar mass
Thus, the concept of molar mass is widely used in chemistry. To describe the process, create polymer complexes and other reactions, it is necessary to calculate this parameter. An important point is to determine the concentration of an active substance in a pharmaceutical substance. For example, using a cell culture, the physiological properties of a new compound are investigated. In addition, molar mass is important in biochemical research. For example, when studying the participation in the metabolic processes of an element. Now the structure of many enzymes is known, so it is possible to calculate their molecular weight, which is mainly measured in kilodaltons (kDa). Today, the molecular weights of almost all components of human blood are known, in particular, hemoglobin. Molecular and molar mass of a substance in certain cases are synonymous. Their differences lie in the fact that the last parameter is the average for all isotopes of the atom.
Any microbiological experiments in the precise determination of the effect of a substance on the enzyme system are carried out using molar concentrations. For example, in biocatalysis and other fields where enzymatic activity research is needed, concepts such as inducers and inhibitors are used. To regulate the activity of the enzyme at the biochemical level, it is necessary to study using precisely molar masses. This parameter has become firmly established in the field of natural and engineering sciences such as physics, chemistry, biochemistry, biotechnology. The processes, characterized in this way, become more understandable from the point of view of mechanisms, determination of their parameters. The transition from fundamental science to applied science is not complete without an indicator of molar mass, starting from physiological solutions, buffer systems and ending with the determination of dosages of pharmaceutical substances for the body.
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Introduction
In the study of chemistry and physics, concepts such as "atom", "relative atomic and molar masses of a chemical element" play an important role. It would seem that nothing new in this area has been discovered for a long time. However, the International Union of Pure and Applied Chemistry (IUPAC) annually updates the values of the atomic masses of chemical elements. Over the past 20 years, the atomic masses of 36 elements have been corrected, and 18 of them have no isotopes.
Taking part in the All-Russian full-time round of the Olympiad in Natural Science, we were offered the following problem: "Suggest a method for determining the molar mass of a substance in a school laboratory."
This task was purely theoretical and I successfully completed it. So I decided experimentally, in a school laboratory, to calculate the molar mass of a substance.
Target:
Determine experimentally the molar mass of a substance in a school laboratory.
Tasks:
Explore the scientific literature that describes how to calculate the relative atomic and molar mass.
Experimentally determine the molar mass of a substance in gaseous and solid states using physical methods.
Draw conclusions.
II. Main part
Basic concepts:
Relative atomic mass is the mass of a chemical element expressed in atomic mass units (amu). For 1 amu 1/12 of the mass of the carbon isotope with an atomic weight of 12. 1 amu = 1.6605655 · 10 -27 kg is taken.
Relative atomic mass - shows how many times the mass of a given atom of a chemical element is more than 1/12 of the mass of the isotope 12 C.
Isotopes- atoms of one chemical element, having a different number of neutrons, and the same number of protons in the nucleus, therefore, having different relative atomic masses.
Molar mass of a substance - this mass of a substance taken in an amount of 1 mol.
1 mol - this is the amount of a substance that contains the same number of atoms (molecules) as there are in 12 g of carbon.
Specific heat of a substance is a physical quantity that shows how much heat must be communicated to a subject weighing 1 kg in order to change its temperature by 1 0 C.
Heat capacity- it is the product of the specific heat capacity of a substance and its mass.
The history of determining the atomic masses of chemical elements:
After analyzing various sources of literature on the history of determining the relative atomic masses of various chemical elements, I decided to summarize the data in a table, which is quite convenient, since in various sources of literature information is given vaguely:
|
Name of scientist, year |
Contribution to the study and determination of relative atomic masses |
Note |
|
John Dalton |
It is clear that it is impossible to weigh the atoms directly. Dalton talked only about "the ratio of the weights of the smallest particles of gaseous and other bodies," that is, about their relative masses. As a unit of mass, Dalton took the mass of a hydrogen atom, and to find the masses of other atoms, he used the percentage compositions of various compounds of hydrogen with other elements found by various researchers. Dalton compiled the world's first table of the relative atomic masses of certain elements. |
|
|
William Prout |
He suggested that all other elements could have arisen from the lightest element, hydrogen, by condensation. In this case, the atomic masses of all elements must be multiples of the mass of the hydrogen atom. For a unit of atomic mass, he suggested choosing hydrogen. |
Only later- over the next few years, it turned out that Prout's hypothesis actually confirmed was: all the elements were actually formed during the explosion of supernovae from the nuclei of hydrogen atoms - protons, as well as neutrons. |
|
1819 Dulong P.I., A.T. Pti: |
Rule of thumb: product of atomic mass and heat capacity- the value is constant. The rule is still used to determine the relative atomic mass of certain substances. |
Berzelius corrected some of the atomic masses of metals on the basis of the rule |
|
Stas, Richards |
Clarification of the relative atomic mass of some elements. |
|
|
S. Ca-Nizzaro |
Determination of the relative atomic mass of some elements by determining the known relative molecular masses of the volatile compounds of the elements |
|
|
Stas, Belgium |
He suggested changing the atomic mass unit and choosing the oxygen atom as the new standard. The mass of the oxygen atom was taken equal to 16,000, the unit of measurement became 1/16 of this mass of oxygen. |
|
|
Complete refutation of Prout's hypothesis based on the determination of the ratio of the masses of chemical elements in some compounds |
||
|
D.I. Mendeleev |
Determined and corrected on the basis of the periodic table the relative atomic masses of some known and not yet discovered chemical elements. |
|
|
The so-called oxygen scale was approved, where the mass of the oxygen atom was taken as the standard. |
||
|
Theodore William Richards |
At the beginning of the 20th century. very accurately determined the atomic masses of 25 chemical elements and corrected mistakes made earlier by other chemists. |
|
|
A mass spectrograph has been created to determine the relative atomic masses |
||
|
For the atomic mass unit (amu), 1/12 of the mass of the carbon isotope 12C (carbon unit) was taken. (1 amu, or 1D (dalton), in SI units of mass is 1.6605710-27 kg.) |
Knowing the relative atomic mass of an atom, you can determine the molar mass of a substance: М = Аr · 10 ³ kg / mol
Methods for determining the molecular weights of elements:
Atomic and molecular weights can be determined either by physical or chemical methods. Chemical methods are distinguished by the fact that at one of the stages they involve not the atoms themselves, but their combinations.
Physical methods:
1 way. Dylogue and Petit's law
In 1819 Dulong together with A.T. Petit, established the law of the heat capacity of solids, according to which the product of the specific heat capacities of simple solids by the relative atomic mass of the constituent elements is an approximately constant value (in modern units of measurement equal to approximately Cv Ar = 25.12 J / (G.K)); now this ratio is called "Dulong-Petit law". The law of specific heat, which for quite a long time remained unnoticed by contemporaries, subsequently served as the basis for a method for an approximate assessment of the atomic masses of heavy elements. From the law of Dulong and Petit, it follows that by dividing 25.12 by the specific heat of a simple substance, which can be easily determined experimentally, one can find an approximate value for the relative atomic mass of a given element. And knowing the relative atomic mass of an element, you can determine the molar mass of a substance.
М = Мr · 10̵ ³ kg / mol
At the initial stage of the development of physics and chemistry, the specific heat of an element was easier to determine than many other parameters, therefore, using this law, approximate values of the RELATIVE ATOMIC MASS were established.
Means, Ar = 25.12 / s
c is the specific heat of the substance
To determine the specific heat of a solid, we will conduct the following experiment:
Pour hot water into the calorimeter and determine its mass and initial temperature.
Let's determine the mass of a solid made from an unknown substance, the relative atomic mass of which we need to determine. We will also determine its initial temperature (its initial temperature is equal to the room temperature of the air, since the body has been in this room for a long time).
Let us lower a solid into the calorimeter with hot water and determine the temperature established in the calorimeter.
Having made the necessary calculation, we determine the specific heat of the solid.
Q1 = c1m1 (t-t1), where Q1 is the amount of heat given off by the water as a result of heat exchange, c1 is the specific heat capacity of water (tabular value), m1 is the mass of water, t is the final temperature, t 1 is the initial temperature of the water, Q2 = c2m2 (t-t2), where Q2 is the amount of heat received by the solid as a result of heat exchange, c2 is the specific heat capacity of the substance (to be determined), m2 is the mass of the substance, t 2 is the initial temperature of the investigated body, since the heat balance equation has the form: Q1 + Q2 = 0 ,
then c2 = c1m1 (t-t1) / (- m2 (t-t2))
|
c, J / (kg 0 K) |
|||||||
Mean relative atomic mass substance turned out
Ar = 26.5 amu
Hence, molar mass a is equal M = 0.0265 kg / mol.
Solid body - aluminum bar
Method 2. Let's calculate the molar mass of air.
Using the equilibrium condition of the system, you can also calculate the molar mass of a substance, for example a gas, for example air.
Fa = F tight(The force of Archimedes acting on the balloon is balanced by the total force of gravity acting on the shell of the balloon, the gas in the balloon, and the load suspended from the balloon.). Of course, considering that the ball is suspended in the air (it does not rise or fall).
Fa- the force of Archimedes, acting on the ball in the air
Fa = ρvg Vsh
ρv - air density
F1- the force of gravity acting on the shell of the ball and the gas (helium) inside the ball
F1 = mobg + mgelg
F2- the force of gravity acting on the load
F2 = mgr g
We get the formula: ρвg Vш= mob g + mgel g + mgr g (1)
Let's use the Mendeleev-Clapeyron formula to calculate the molar mass of air:
Let us express the molar mass of air:
In equation (3) we substitute equation (2) instead of air density. So, we got the formula for calculating the molar mass of air:
Therefore, to find the molar mass of air, you need to measure:
1) weight of cargo
2) the mass of helium
3) shell mass
4) air temperature
5) air pressure (atmospheric pressure)
6) the volume of the ball
R- universal gas constant, R = 8.31 J / (mol K)
Barometer showed atmospheric pressure
equal pa = 96000Pa
Indoor air temperature:
T = 23 + 273 = 297K
We determined the mass of the load and the mass of the shell of the ball using electronic scales:
mgr = 8.02g
ball shell mass:
mob = 3.15g
We determined the volume of the ball in two ways:
a) our ball turned out to be round. Having measured the circumference of the ball in several places, we determined the radius of the ball. And then its volume: V = 4/3 · πR³
L = 2πR, Lav = 85.8cm = 0.858m, therefore R = 0.137m
Vsh = 0.0107m³
b) poured water into a bucket to the very edge, after placing it in a tray to drain the water. We lowered the balloon completely into the water, part of the water poured into the bath under the bucket, measuring the volume of water poured out of the bucket, we determined the volume of the balloon: Vwater = Vsh = 0.011m³
(The ball in the picture was closer to the camera, so it seems larger)
So, for the calculation, we took the average value of the volume of the ball:
Vsh = 0.0109m³
We determine the mass of helium in using the Mendeleev-Clapeyron equation, taking into account that the temperature of helium is equal to the temperature of the air, and the pressure of helium inside the ball is equal to atmospheric.
Molar mass of helium 0.004 kg / mol:
mgel = 0.00169 kg
Substituting all measurement results into formula (4), we obtain the value of the molar mass of air:
M = 0.030 kg / mol
(table value of molar mass
air 0.029 kg / mol)
Output: in a school laboratory, you can determine by physical methods the relative atomic mass of a chemical element and the molar mass of a substance. Having done this work, I learned a lot about the methods of determining the relative atomic mass. Of course, many methods are not available for a school laboratory, but, nevertheless, even using elementary equipment, I was able to experimentally determine by physical methods the relative atomic mass of a chemical element and the molar mass of a substance. Therefore, I have fulfilled the goal and tasks set in this work.
List of used literature
alhimik.ru
alhimikov.net
https://ru.wikipedia.org/wiki/Molar_mass
G.I.Deryabina, G.V. Kantaria. 2.2 Mole, molar mass. Organic chemistry: webbook.
http://kf.info.urfu.ru/glavnaja/
https://ru.wikipedia.org/wiki/Molar_mass h
In lesson 5 “ Mole and molar mass"From the course" Chemistry for dummies»Consider a mole as a unit for measuring the amount of a substance; we will give a definition of Avogadro's number, as well as learn how to determine the molar mass and solve problems for the amount of matter. This lesson will be based on the chemistry fundamentals outlined in previous lessons, so if you are learning chemistry from scratch, I recommend that you skim through them at least briefly.
Prior to this lesson, we only discussed individual molecules and atoms, and we expressed their masses in atomic mass units. In real life, it is impossible to work with individual molecules, because they are negligible. For this, chemists weigh the substances not in amu, but in grams.
To change from the molecular mass scale to the laboratory scale, use unit for measuring the amount of a substance called a mole. 1 mole contains 6.022 · 10 23 particles (atoms or molecules) and is a dimensionless quantity. The number 6.022 · 10 23 is called, which is defined as the number of particles contained in 12 g of carbon atoms 12 C. It is important to understand that 1 mol of any substance always contains the same number of particles (6.022 · 10 23).
As already mentioned, the term "mole" is applied not only to molecules but also to atoms. For example, if you are talking about a mole of helium (He), then this means that you have an amount equal to 6.022 · 10 23 atoms. Likewise, 1 mole of water (H 2 O) implies an amount equal to 6.022 · 10 23 molecules. Most often, however, the mole is applied specifically to molecules.
Molar mass Is the mass of 1 mole of a substance, expressed in grams. The molar mass of one mole of any chemical element can be easily found from the periodic table, since the molar mass is numerically equal to the atomic mass, but their dimensions are different (molar mass has the dimension g / mol). Write down and memorize the formulas for calculating the molar mass, amount of substance and number of molecules:
- Molar mass formula M = m / n
- Amount of substance formula n = m / M
- Number of molecules formula N = N A n
where m- the mass of the substance, n- the amount of substance (number of moles), M- molar mass, N- the number of molecules, N A Is Avogadro's number. Because of the molar mass of a substance, chemists can count atoms and molecules in the laboratory simply by weighing them. This makes it convenient to use the concept mole.
The figure shows four flasks with different substances, but each of them contains only 1 mole of a substance. You can double-check using the formulas above.
Tasks for the amount of substance
Example 1. How many grams of H 2, H 2 O, CH 3 OH, octane (C 8 H 18) and neon gas (Ne) are there in 1 mole?
Solution: The molecular weights (in atomic mass units) of the listed substances are given in the periodic table. 1 mol of each of the named substances has the following mass:
Since the masses indicated in the solution of example 1 give the correct relative masses of the weighed molecules, the indicated mass of each of the listed substances contains the same number of molecules. This makes it convenient to use the concept of praying. It is not even necessary to know what the numerical value of the mole is equal to, although we already know that it is 6.022 · 10 23; this quantity is called Avogadro's number and is denoted by the symbol N A. The transition from individual molecules to moles means an increase in the measurement scale 6.022 × 10 23 times. Avogadro's number is also a multiplier for converting atomic mass units to grams: 1 g = 6.022 · 10 23 amu. If we understand the molecular weight as the mass of a mole of a substance, then it should be measured in grams per mole; if we really mean the mass of one molecule, then it numerically coincides
with the molecular weight of a substance, but expressed in atomic mass units per molecule. Both ways of expressing molecular weight are correct.
Example 2. How many moles are and how many molecules contain 8 g of gaseous oxygen O 2?
Solution: We write out the atomic mass of the oxygen atom (O) from the periodic table, which is 15.99 amu, round up to 16. Since we have an oxygen molecule consisting of two O atoms, its atomic mass is 16 × 2 = 32 a.u. Okay, now let's convert it to molar mass: 32 amu = 32 g / mol. This means that 1 mol (6.022 · 10 23 molecules) of O 2 has a mass of 32 grams. Well, in conclusion, using the formulas above, we find the amount of substance (mol) and the number of molecules contained in 8 grams of O 2:
- n = m / M = 8g / 32g / mol = 0.25 mol
- N = N A × n = 6.022 10 23 × 0.25 = 1.505 10 23 molecules
Example 3. 1 molecule of Н 2 reacts with 1 molecule of Сl 2, as a result of which 2 molecules of gaseous hydrogen chloride HCl are formed. How much chlorine gas needs to be used to fully react with 1 kilogram (kg) of hydrogen gas?
Solution: The molecular weights of H 2 and Cl 2 are 2.0160 and 70.906 g / mol, respectively. Therefore, 1000 g of H 2 contains
Without even finding out how many molecules are contained in one mole of a substance, we can be sure that 496 moles of Cl 2 contain the same number of molecules as 496.0 moles, or 1000 g, of H 2. How many grams of Cl 2 is contained in 496 moles of this substance? Since the molecular weight of Cl 2 is 70.906 g / mol, then
Example 4. How many H 2 and Cl 2 molecules are involved in the reaction described in example 3?
Solution: 496 moles of any substance should contain 496 moles × 6.022 10 23 molecules / mol, which is equal to 2.99 · 10 26 molecules.
To clearly show how large Avogadro's number is, let us give the following example: 1 mole of coconuts each with a diameter of 14 centimeters (cm) could fill such a volume as our planet Earth occupies. The use of moles in chemical calculations is discussed in the next chapter, but the idea of this had to be introduced already here, since we need to know how the transition from the molecular scale for measuring mass to the laboratory scale is carried out.
Hopefully lesson 5 " Mole and molar mass"Was informative and understandable. If you have any questions, write them in the comments.
In chemistry, the values of the absolute masses of molecules are not used, but the value of the relative molecular weight is used. It shows how many times the mass of a molecule is more than 1/12 of the mass of a carbon atom. This value is designated M r.
The relative molecular weight is equal to the sum of the relative atomic masses of its constituent atoms. Let's calculate the relative molecular weight of water.
You know that a water molecule contains two hydrogen atoms and one oxygen atom. Then its relative molecular mass will be equal to the sum of the products of the relative atomic mass of each chemical element by the number of its atoms in a water molecule:
Knowing the relative molecular masses of gaseous substances, one can compare their densities, i.e., calculate the relative density of one gas by another - D (A / B). The relative density of gas A over gas B is equal to the ratio of their relative molecular masses:
Let's calculate the relative density of carbon dioxide by hydrogen:
Now we calculate the relative density of carbon dioxide by hydrogen:
D (coal year / hydrogen) = M r (coal year): M r (hydrogen) = 44: 2 = 22.
Thus, carbon dioxide is 22 times heavier than hydrogen.
As you know, Avogadro's law applies only to gaseous substances. But chemists need to have an idea of the number of molecules and in portions of liquid or solid substances. Therefore, to compare the number of molecules in substances, chemists introduced the value - molar mass .
Molar mass is denoted M, it is numerically equal to the relative molecular weight.
The ratio of the mass of a substance to its molar mass is called the amount of substance .
The amount of substance is indicated n... This is a quantitative characteristic of a portion of a substance, along with mass and volume. The amount of substance is measured in moles.
The word "mole" comes from the word "molecule". The number of molecules in equal amounts of a substance is the same.
It has been experimentally established that 1 mole of a substance contains particles (for example, molecules). This number is called Avogadro's number. And if you add a unit of measurement to it - 1 / mol, then it will be a physical quantity - Avogadro's constant, which is denoted by N A.
Molar mass is measured in g / mol. The physical meaning of molar mass is that this mass is 1 mole of a substance.
According to Avogadro's law, 1 mole of any gas will occupy the same volume. The volume of one mole of gas is called the molar volume and is denoted by V n.
Under normal conditions (which is 0 ° C and normal pressure is 1 atm. Or 760 mm Hg. Or 101.3 kPa), the molar volume is 22.4 l / mol.
Then the amount of substance of the gas at n.u. can be calculated as the ratio of gas volume to molar volume.
PROBLEM 1... What amount of substance corresponds to 180 g of water?
OBJECTIVE 2. Let's calculate the volume at standard conditions, which will be occupied by carbon dioxide in the amount of 6 mol.
Bibliography
- Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky et al. "Chemistry, grade 8" / P.А. Orzhekovsky, N.A. Titov, F.F. Hegel. - M .: AST: Astrel, 2006. (p. 29-34)
- Ushakova O.V. Chemistry workbook: grade 8: to the textbook by P.A. Orzhekovsky and others. "Chemistry. Grade 8 "/ О.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M .: AST: Astrel: Profizdat, 2006. (p. 27-32)
- Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M .: AST: Astrel, 2005. (§§ 12, 13)
- Chemistry: nonorg. chemistry: textbook. for 8 cl. general institution / G.E. Rudzitis, F.G. Feldman. - M .: Education, JSC "Moscow textbooks", 2009. (§§ 10, 17)
- Encyclopedia for children. Volume 17. Chemistry / Chap. ed. by V.A. Volodin, led. scientific. ed. I. Leenson. - M .: Avanta +, 2003.
- A single collection of digital educational resources ().
- Electronic version of the journal "Chemistry and Life" ().
- Chemistry tests (online) ().
Homework
1.p.69 No. 3; p.73 No. No. 1, 2, 4 from the textbook "Chemistry: 8th grade" (PA Orzhekovsky, LM Meshcheryakova, LS Pontak. M .: AST: Astrel, 2005).
2. №№ 65, 66, 71, 72 from the Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky et al. "Chemistry, grade 8" / P.А. Orzhekovsky, N.A. Titov, F.F. Hegel. - M .: AST: Astrel, 2006.
Molecular weight is one of the basic concepts in modern chemistry. Its introduction became possible after the scientific substantiation of Avogadro's assertion that many substances consist of tiny particles - molecules, each of which, in turn, consists of atoms. Science owes this judgment largely to the Italian chemist Amadeo Avogadro, who scientifically substantiated the molecular structure of substances and gave chemistry many important concepts and laws.
Mass units of elements
Initially, the hydrogen atom was taken as the base unit of atomic and molecular mass as the lightest element in the Universe. But the atomic masses for the most part were calculated on the basis of their oxygen compounds, so it was decided to choose a new standard for determining atomic masses. The atomic mass of oxygen was taken equal to 15, the atomic mass of the lightest substance on Earth, hydrogen, - 1. In 1961, the oxygen system for determining the weight was generally accepted, but created certain inconveniences.
In 1961, a new scale of relative atomic masses was adopted, the standard for which was the carbon isotope 12 C. The atomic mass unit (abbreviated amu) is 1/12 of the mass of this standard. At present, atomic mass is the mass of an atom, which must be expressed in amu.
Molecule mass
The mass of a molecule of any substance is equal to the sum of the masses of all atoms that form a given molecule. The lightest molecular weight of a gas is hydrogen, its compound is written as H 2 and has a value close to two. A water molecule consists of an oxygen atom and two hydrogen atoms. This means that its molecular weight is 15.994 + 2 * 1.0079 = 18.0152 amu. Complex organic compounds - proteins and amino acids - have the largest molecular weights. The molecular weight of a protein structural unit ranges from 600 to 10 6 and more, depending on the number of peptide chains in this macromolecular structure.
Moth
Along with the standard units of mass and volume in chemistry, a completely special system unit is used - the mole.
A mole is the amount of a substance that contains as many structural units (ions, atoms, molecules, electrons) as is contained in 12 grams of the 12 C isotope.
When applying a measure of the amount of a substance, it is necessary to indicate which structural units are meant. As follows from the concept of "mole", in each individual case it is necessary to indicate exactly what structural units are in question - for example, the mole of H + ions, the mole of H 2 molecules, and so on.
Molar and molecular weight
The mass of the amount of a substance in 1 mol is measured in g / mol and is called the molar mass. The relationship between molecular and molar mass can be written as the equation
ν = k × m / M, where k is the coefficient of proportionality.
It is easy to say that for any ratios the proportionality coefficient will be equal to one. Indeed, the isotope of carbon has a relative molecular mass of 12 amu, and, according to the definition, the molar mass of this substance is 12 g / mol. The ratio of molecular weight to molar is 1. Hence, we can conclude that the molar and molecular weight have the same numerical values.
Gas volumes
As you know, all substances around us can be in a solid, liquid or gaseous state of aggregation. For solids, the most common base measure is mass, for solids and liquids, volume. This is due to the fact that solids retain their shape and finite dimensions, Liquid and gaseous substances do not have finite dimensions. The peculiarity of any gas is that between its structural units - molecules, atoms, ions - the distance is many times greater than the same distance in liquids or solids. For example, under normal conditions one mole of water takes up a volume of 18 ml - approximately the same amount fits in one tablespoon. The volume of one mole of fine-crystalline table salt is 58.5 ml, and the volume of 1 mole of sugar is 20 times more than a mole of water. Even more space is required for gases. Under normal conditions, one mole of nitrogen takes up a volume 1240 times greater than one mole of water.
Thus, the volumes of gaseous substances differ significantly from the volumes of liquid and solid. This is due to the difference in distances between molecules of substances in different states of aggregation.
Normal conditions
The state of any gas is highly dependent on temperature and pressure. For example, nitrogen at a temperature of 20 ° C takes up a volume of 24 liters, and at 100 ° C at the same pressure - 30.6 liters. Chemists took into account this dependence, so it was decided to reduce all operations and measurements with gaseous substances to normal conditions. All over the world, the parameters of normal conditions are the same. For gaseous chemicals, these are:
- Temperature at 0 ° C.
- Pressure 101.3 kPa.
For normal conditions, a special abbreviation is adopted - n.o. Sometimes in problems this designation is not written, then you should carefully re-read the conditions of the problem and bring the given gas parameters to normal conditions.
Calculation of the volume of 1 mole of gas
As an example, it is easy to calculate one mole of any gas, such as nitrogen. To do this, you first need to find the value of its relative molecular weight:
M r (N 2) = 2 × 14 = 28.
Since the relative molecular mass of a substance is numerically equal to the molar mass, then M (N 2) = 28 g / mol.
It has been experimentally found that under normal conditions the density of nitrogen is 1.25 g / liter.
Substitute this value into the standard formula known from the school physics course, where:
- V is the gas volume;
- m is the mass of the gas;
- ρ is the density of the gas.
We get that the molar volume of nitrogen under normal conditions
V (N 2) = 25 g / mol: 1.25 g / liter = 22.4 L / mol.
It turns out that one mole of nitrogen takes 22.4 liters.
If you perform such an operation with all existing gaseous substances, you can come to an amazing conclusion: the volume of any gas under normal conditions is 22.4 liters. Regardless of what kind of gas we are talking about, what is its structure and physicochemical characteristics, one mole of this gas will occupy a volume of 22.4 liters.
The molar volume of a gas is one of the most important constants in chemistry. This constant makes it possible to solve many chemical problems associated with measuring the properties of gases under normal conditions.
Outcomes
The molecular weight of gaseous substances is important for determining the amount of a substance. And if a researcher knows the amount of substance of a particular gas, he can determine the mass or volume of such a gas. For the same portion of a gaseous substance, the following conditions are simultaneously fulfilled:
ν = m / M ν = V / V m.
If we remove the constant ν, we can equalize these two expressions:
So you can calculate the mass of one portion of the substance and its volume, and also the molecular weight of the investigated substance becomes known. Using this formula, you can easily calculate the volume-to-mass ratio. When this formula is reduced to the form M = m V m / V, the molar mass of the desired compound will become known. In order to calculate this value, it is enough to know the mass and volume of the investigated gas.
It should be remembered that a strict correspondence of the real molecular weight of a substance to that found by the formula is impossible. Any gas contains a lot of impurities and additives that make certain changes in its structure and affect the determination of its mass. But these fluctuations introduce changes in the third or fourth decimal place in the result found. Therefore, for school problems and experiments, the results found are quite plausible.