Molar mass is a definition in physics. Start in science

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Introduction

When studying chemistry and physics important role play such concepts as "atom", "relative atomic and molar masses chemical element". It would seem that nothing new has been discovered in this area for a long time. However, international union Theoretical and Applied Chemistry (IUPAC) annually refines the values of the atomic masses of chemical elements. Over the past 20 years, the atomic masses of 36 elements have been corrected, 18 of which have no isotopes.

Taking part in the All-Russian full-time round of the Olympiad in Natural Science, we were offered the following task: “Suggest a method for determining the molar mass of a substance under conditions school laboratory».

This task was purely theoretical and I successfully coped with it. So I decided experimentally, in the conditions of 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 scientific literature, which describes how to calculate relative atomic and molar masses.

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 (a.m.u.). For 1 amu 1/12 of the mass of a carbon isotope with an atomic weight of 12 is adopted. 1 amu = 1.6605655 10 -27 kg.

Relative atomic mass - shows how many times the mass of a given atom of a chemical element is greater than 1/12 of the mass of the 12 C isotope.

isotopes- atoms of one chemical element having different amount neutrons, and the same number of protons in the nucleus, therefore, having different relative atomic masses.

The molar mass of a substance is 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 capacity of a substance- This is a physical quantity that shows how much heat it is necessary to impart to a topic with a mass of 1 kg in order to change its temperature by 1 0 C.

Heat capacity- 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, because. V various sources Literature information is given vaguely:

Full name of the scientist, year	Contribution to the study and determination of relative atomic masses	Note
John Dalton	It is clear that it is impossible to directly weigh the atoms. 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 the 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 different researchers. Dalton compiled the world's first table of the relative atomic masses of certain elements.
William Prout	He suggested that from the lightest element - hydrogen, all other elements could arise by condensation. In this case, the atomic masses of all elements must be a multiple of the mass of the hydrogen atom. For a unit of atomic mass, he proposed to choose hydrogen.	Only in the next over the years, it turned out that Prout's hypothesis actually confirmed las: all elements were really formed during the explosion of supernovae from the nuclei of hydrogen atoms - protons, as well as neutrons.
1819 Dulong P.I., A.T. Petit:	Rule of thumb: product of atomic mass and heat capacity- the value is constant. The rule is still used today to determine the relative atomic mass of certain substances	Berzelius, on the basis of the rule, corrected some atomic masses of metals
Stas, Richards	Refinement of the relative atomic mass of some elements.
S. Cannizzaro	Determining the relative atomic mass of certain elements by determining the known relative molecular masses of the volatile compounds of the elements
Stas, Belgium	He proposed to change the atomic mass unit and choose the oxygen atom as the new standard. The mass of an oxygen atom was taken equal to 16,000 units of measurement and became 1/16 of this mass of oxygen.
	Complete refutation of the Prout hypothesis based on the determination of the ratio of the masses of chemical elements in some compounds
D.I. Mendeleev	Based on the periodic table, he determined and corrected the relative atomic masses of some known and not yet discovered chemical elements.
	The so-called oxygen scale was approved, where the mass of an oxygen atom was taken as a standard
Theodore William Richards	At the beginning of the 20th century very accurately determined the atomic masses of 25 chemical elements and corrected the mistakes made earlier by other chemists.
	Created a mass spectrograph to determine the relative atomic masses
	The atomic mass unit (a.m.u.) was taken to be 1/12 of the mass of the 12C carbon isotope (carbon unit). (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: M \u003d Ar 10̄ ³ kg / mol

Methods for determining the molecular weights of elements:

Atomic and molecular weight 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. Dulog and Petit 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 and the relative atomic mass of the constituent elements is an approximately constant value (in modern units, equal to approximately Cv Ar = 25.12 J/(g.K)); now this ratio is called the Dulong-Petit law. The law of specific heat capacity, quite for a long time remained unnoticed by contemporaries, later served as the basis for the method of approximate estimation of the atomic masses of heavy elements. From the law of Dulong and Petit it follows that dividing 25.12 by the specific heat a simple substance, easily determined experimentally, one can find the approximate value of the relative atomic mass of a given element. And knowing the relative atomic mass of the element, you can determine the molar mass of the substance.

M \u003d Mr 10̵ ³ kg / mol

At the initial stage of the development of physics and chemistry, the specific heat capacity of an element was easier to determine than many other parameters, therefore, using this law, the approximate values \u200b\u200bof RELATIVE ATOMIC MASS were established.

Means, Ar=25.12/s

c is the specific heat capacity of the substance

To determine the specific heat capacity of a solid body, we will conduct the following experiment:

Q1=c1m1(t-t1), where Q1 is the amount of heat given off by water as a result of heat transfer, c1 is the specific heat capacity of water (table 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 transfer, c2 is the specific heat of the substance (to be determined), m2 is the mass of the substance, t 2 is the initial temperature of the body under study, because the heat balance equation has the form: Q1 + Q2 = 0 ,

Then c2 = c1m1(t-t1) /(- m2(t-t2))

						s, J/ (kg 0 K)

Average value relative atomic mass substances turned out

Ar = 26.5 a.m.u.

Hence, molar mass a is equal to M \u003d 0.0265 kg / mol.

Solid body - aluminum bar

2 way. Calculate the molar mass of air.

Using the equilibrium condition of the system, you can also calculate the molar mass of a substance, such as a gas, such as air.

Fa = Fstrand(Archimedes force acting on balloon balanced total force gravity acting on the shell of the ball, the gas in the ball, and the load suspended from the ball.). Of course, given that the ball is hovering in the air (it does not rise or fall).

Fa- the Archimedes force acting on the ball in the air

Fa \u003d ρvg Vsh

ρv - air density

F1- gravity acting on the shell of the ball and the gas (helium) inside the ball

F1=mob g + mgel g

F2 is the force of gravity acting on the load

F2=mg g

We get the formula: ρing Vsh= mob g + mgel g + mg g (1)

Let's use the Mendeleev-Clapeyron formula to calculate the molar mass of air:

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) mass of cargo

2) mass of helium

3) shell mass

4) air temperature

5) air pressure (atmospheric pressure)

6) the volume of the ball

R is the universal gas constant, R=8.31 J/(mol K)

Barometer showed atmospheric pressure

equal pa = 96000Pa

Air temperature in the room:

T=23 +273=297K

We determined the mass of the load and the mass of the shell of the ball using electronic scales:

mg = 8.02g

ball shell mass:

mo = 3.15g

We determined the volume of the sphere in two ways:

a) our ball turned out to be round. By measuring 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, hence R=0.137m

Vsh= 0.0107m³

b) poured water into the bucket to the very edge, after placing it with a bath to drain the water. We lowered the ball 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 \u003d Vsh \u003d 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 helium mass in using the Mendeleev-Clapeyron equation, given that the helium temperature is equal to the air temperature, and the helium pressure 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)

Conclusion: in a school laboratory, it is possible to 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 how to determine relative atomic mass. Of course, many methods are not available for the school laboratory, but, nevertheless, even using elementary equipment, I was able to experimentally by physical means determine the relative atomic mass of a chemical element and the molar mass of a substance. Therefore, I fulfilled the purpose and tasks set in this work.

List of used literature

alchemik.ru

alhimikov.net

https://ru.wikipedia.org/wiki/Molar_mass

G. I. Deryabina, G. V. Kantaria. 2.2. Mole, molar mass. Organic Chemistry: A Web Textbook.

http://kf.info.urfu.ru/glavnaja/

https://ru.wikipedia.org/wiki/Molar_mass h

And the ability to do calculations, of course. For example, a well-known substance is sulfuric. It finds so wide in a variety of industries that it rightfully bears the name "chemistry". What is her?

Write the exact formula for sulfuric acid: H2SO4. Now take the periodic table and see what are the atomic masses of all the elements that make up it. These three elements are hydrogen, sulfur and oxygen. The atomic mass of hydrogen is 1, sulfur - 32, oxygen - 16. Therefore, the total molecular weight of sulfuric acid, taking into account the indices, is: 1 * 2 + 32 + 16 * 4 = 98 amu (atomic mass units).

And now let's remember one more mole: this is the amount substances, whose mass is numerically equal to its mass expressed in atomic units. Thus, it turns out that 1 mole of sulfuric acid weighs 98 grams. Here is its molar mass. Problem solved.

Suppose you are given the following conditions: there are 800 milliliters of a 0.2 molar solution (0.2 M) of some salt, and it is known that in dry form this salt weighs 25 grams. It is required to calculate its molar mass.

First, remember the definition of a 1-molar (1M) solution. This is a solution in which 1 contains 1 mole of any substances. Accordingly, 1 liter of a 0.2M solution would contain 0.2 mol substances. But you have not 1 liter, but 0.8 liters. Therefore, in fact, you have 0.8 * 0.2 = 0.16 mol substances.

And then everything becomes easier than ever. If 25 grams of salt, according to the conditions of the problem, are 0.16 moles, what is the amount equal to one mole? Having made the calculation in one step, you will find: 25 / 0.16 \u003d 156.25 grams. The molar mass of salt is 156.25 grams/mol. Problem solved.

In your calculations, you used rounded values for the atomic weights of hydrogen, sulfur, and oxygen. If you want to make calculations with high precision, rounding is not allowed.

Sources:

molar mass of salt
Calculating the molar mass equivalent

The masses of atoms or molecules are extremely small, therefore, in molecular physics, instead of the masses of molecules and atoms themselves, it is customary to use, at the suggestion of Dalton, their relative values, comparing mass a molecule or atom with 1/12 of the mass of a carbon atom. The amount of a substance that contains as many molecules or atoms as there are in 12 grams of carbon is called a mole. The molar mass of a substance (M) is the mass of one mole. Molar mass is a scalar quantity, it is measured in international system SI in kilograms divided by moles.

Instruction

To calculate the molar mass it is enough to know two quantities: mass(m), expressed in kilograms, and the amount of substance (v), measured in moles, substituting them into the formula: M \u003d m / v.
Example. Let it be necessary to determine the molar mass 100 g of water in 3 moles. To do this, you must first mass water in from grams - 100g \u003d 0.01kg. Next, substitute the values \u200b\u200bin the formula, for the molar: M \u003d m / v \u003d 0.01 kg / 3 mol \u003d 0.003 kg / mol.

Molecular physics studies the properties of bodies, guided by the behavior of individual molecules. All visible processes take place at the level of interaction of the smallest particles, what we see with the naked eye is only a consequence of these subtle deep connections.

In contact with

Basic concepts

Molecular physics is sometimes seen as a theoretical extension of thermodynamics. Originating much earlier, thermodynamics was engaged in the study of the transfer of heat into work, pursuing purely practical goals. She did not produce a theoretical substantiation, describing only the results of experiments. The basic concepts of molecular physics emerged later, in the 19th century.

She studies the interaction of bodies on molecular level, guided by the statistical method, which determines the patterns in the chaotic movements of minimal particles - molecules. Molecular physics and thermodynamics complement each other, considering processes with different points vision. At the same time, thermodynamics does not concern atomic processes, dealing only with macroscopic bodies, while molecular physics, on the contrary, considers any process precisely from the point of view of the interaction of individual structural units.

All concepts and processes have their own designations and are described by special formulas that most clearly represent the interactions and dependencies of certain parameters on each other. Processes and phenomena intersect in their manifestations, different formulas can contain the same quantities and be expressed in different ways.

Amount of substance

The amount of a substance determines the relationship between (mass) and the number of molecules that this mass contains. The fact is that different substances with the same mass different number minimal particles. The processes taking place at the molecular level can only be understood by considering the number of atomic units involved in the interactions. Unit of measure for the amount of a substance, adopted in the SI system, - mol.

Attention! One mole always contains the same number of minimal particles. This number is called Avogadro's number (or constant) and is equal to 6.02×1023.

This constant is used in cases where calculations require taking into account the microscopic structure of a given substance. Dealing with the number of molecules is difficult, since you have to operate with huge numbers, so the mole is used - a number that determines the number of particles per unit mass.

The formula for determining the amount of a substance:

The calculation of the amount of substance is carried out in different occasions, is used in many formulas and is important in molecular physics.

Gas pressure

Gas pressure is an important quantity that has not only theoretical but also practical significance. Consider the formula for gas pressure used in molecular physics, with explanations necessary for a better understanding.

To formulate the formula, some simplifications will have to be made. Molecules are complex systems having a multistage structure. For simplicity, we consider gas particles in a certain vessel as elastic homogeneous balls that do not interact with each other (ideal gas).

The speed of motion of minimal particles will also be assumed to be the same. By introducing such simplifications that do not change the true situation much, we can derive the following definition: gas pressure is the force exerted by the impacts of gas molecules on the walls of vessels.

At the same time, taking into account the three-dimensionality of space and the presence of two directions of each dimension, it is possible to limit the number of structural units acting on the walls as 1/6 part.

Thus, bringing together all these conditions and assumptions, we can deduce gas pressure formula under ideal conditions.

The formula looks like this:

where P - gas pressure;

n is the concentration of molecules;

K- Boltzmann's constant(1.38×10-23);

Ek - gas molecules.

There is another version of the formula:

P = nkT,

where n is the concentration of molecules;

T is the absolute temperature.

Gas volume formula

The volume of a gas is the space that a given amount of gas occupies under certain conditions. Unlike solids, which have a constant volume, practically independent of environmental conditions, gas can change volume with pressure or temperature.

The gas volume formula is the Mendeleev-Clapeyron equation, which looks like this:

PV=nRT

where P - gas pressure;

V is the volume of gas;

n is the number of moles of gas;

R is the universal gas constant;

T is the gas temperature.

By simple permutations, we obtain the formula for the volume of gas:

Important! According to Avogadro's law, equal volumes of any gases placed in exactly the same conditions - pressure, temperature - will always contain an equal number of minimal particles.

Crystallization

Crystallization is a phase transition of a substance from a liquid to a solid state, i.e. the reverse process of melting. The process of crystallization occurs with the release of heat, which is required to be removed from the substance. The temperature coincides with the melting point, the whole process is described by the formula:

Q = λm,

where Q is the amount of heat;

λ - heat of fusion;

This formula describes both crystallization and melting, since they are, in fact, two sides of the same process. For a substance to crystallize, must be cooled down to melting temperature., and then remove the amount of heat equal to the product of mass and specific heat melting point (λ). During crystallization, the temperature does not change.

There is another way to understand this term - crystallization from supersaturated solutions. In this case, the reason for the transition is not only the achievement of a certain temperature, but also the degree of saturation of the solution with a certain substance. At a certain stage, the number of solute particles becomes too large, which causes the formation of small single crystals. They attach molecules from solution, producing layer-by-layer growth. Depending on the growth conditions, the crystals have different shapes.

Number of molecules

It is easiest to determine the number of particles contained in a given mass of a substance using the following formula:

It follows that the number of molecules is equal to:

That is, it is necessary first of all to determine the amount of substance per certain mass. Then it is multiplied by the Avogadro number, resulting in the number of structural units. For compounds, the calculation is carried out by summing the atomic weight of the components. Consider a simple example:

Determine the number of water molecules in 3 grams. The formula (H2O) contains two atoms and one . The total atomic weight of the minimum particle of water will be: 1+1+16 = 18 g/mol.

Amount of substance in 3 grams of water:

Number of molecules:

1/6 x 6 x 1023 = 1023.

Molecule mass formula

One mole always contains the same number of minimal particles. Therefore, knowing the mass of a mole, we can divide it by the number of molecules (Avogadro's number), resulting in the mass of a system unit.

It should be noted that this formula applies only to inorganic molecules. Organic molecules are much larger, their magnitude or weight have completely different meanings.

Molar mass of gas

molar mass is mass in kilograms of one mole of a substance. Since one mole contains the same number of structural units, the molar mass formula looks like this:

M = κ × Mr

where k is the coefficient of proportionality;

Mr is the atomic mass of the substance.

The molar mass of a gas can be calculated using the Mendeleev-Clapeyron equation:

pV=mRT/M,

from which you can deduce:

M=mRT/pV

Thus, the molar mass of a gas is directly proportional to the product of the gas mass times the temperature and the universal gas constant, and inversely proportional to the product of the gas pressure and its volume.

Attention! It should be noted that the molar mass of a gas as an element may differ from a gas as a substance, for example, the molar mass of the element oxygen (O) is 16 g/mol, and the mass of oxygen as a substance (O2) is 32 g/mol.

Basic provisions of the ICT.

Physics in 5 minutes - molecular physics

Conclusion

The formulas contained in molecular physics and thermodynamics make it possible to calculate the quantitative values of all processes that occur with solids and gases. Such calculations are necessary both in theoretical research and in practice, since they contribute to the solution of practical problems.

One of the basic units in the International System of Units (SI) is the unit of quantity of a substance is the mole.

mole – this is such an amount of a substance that contains as many structural units of a given substance (molecules, atoms, ions, etc.) as there are carbon atoms in 0.012 kg (12 g) of a carbon isotope 12 WITH .

Given that the value of the absolute atomic mass for carbon is m(C) \u003d 1.99 10  26 kg, you can calculate the number of carbon atoms N A contained in 0.012 kg of carbon.

A mole of any substance contains the same number of particles of this substance (structural units). The number of structural units contained in a substance with an amount of one mole is 6.02 10 23 and called Avogadro's number (N A ).

For example, one mole of copper contains 6.02 10 23 copper atoms (Cu), and one mole of hydrogen (H 2) contains 6.02 10 23 hydrogen molecules.

molar mass(M) is the mass of a substance taken in an amount of 1 mol.

The molar mass is denoted by the letter M and has the unit [g/mol]. In physics, the dimension [kg/kmol] is used.

In the general case, the numerical value of the molar mass of a substance numerically coincides with the value of its relative molecular (relative atomic) mass.

For example, the relative molecular weight of water is:

Mr (H 2 O) \u003d 2Ar (H) + Ar (O) \u003d 2 ∙ 1 + 16 \u003d 18 a.m.u.

The molar mass of water has the same value, but is expressed in g/mol:

M (H 2 O) = 18 g/mol.

Thus, a mole of water containing 6.02 10 23 water molecules (respectively 2 6.02 10 23 hydrogen atoms and 6.02 10 23 oxygen atoms) has a mass of 18 grams. 1 mole of water contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms.

1.3.4. The relationship between the mass of a substance and its quantity

Knowing the mass of a substance and its chemical formula, and hence the value of its molar mass, one can determine the amount of a substance and, conversely, knowing the amount of a substance, one can determine its mass. For such calculations, you should use the formulas:

where ν is the amount of substance, [mol]; m is the mass of the substance, [g] or [kg]; M is the molar mass of the substance, [g/mol] or [kg/kmol].

For example, to find the mass of sodium sulfate (Na 2 SO 4) in the amount of 5 mol, we find:

1) the value of the relative molecular weight of Na 2 SO 4, which is the sum of the rounded values of the relative atomic masses:

Mr (Na 2 SO 4) \u003d 2Ar (Na) + Ar (S) + 4Ar (O) \u003d 142,

2) the value of the molar mass of the substance numerically equal to it:

M (Na 2 SO 4) = 142 g/mol,

3) and, finally, a mass of 5 mol of sodium sulfate:

m = ν M = 5 mol 142 g/mol = 710 g

Answer: 710.

1.3.5. The relationship between the volume of a substance and its quantity

Under normal conditions (n.o.), i.e. at pressure R , equal to 101325 Pa (760 mm Hg), and temperature T, equal to 273.15 K (0 С), one mole of various gases and vapors occupies the same volume, equal to 22.4 l.

The volume occupied by 1 mole of gas or vapor at n.o. is called molar volumegas and has the dimension of a liter per mole.

V mol \u003d 22.4 l / mol.

Knowing the amount of gaseous substance (ν ) And molar volume value (V mol) you can calculate its volume (V) under normal conditions:

V = ν V mol,

where ν is the amount of substance [mol]; V is the volume of the gaseous substance [l]; V mol \u003d 22.4 l / mol.

Conversely, knowing the volume ( V) of a gaseous substance under normal conditions, you can calculate its amount (ν) :

Molecular weight is one of the basic concepts in modern chemistry. Its introduction became possible after the scientific substantiation of Avogadro's statement that many substances consist of the smallest particles - molecules, each of which, in turn, consists of atoms. Science owes much of this judgment to the Italian chemist Amadeo Avogadro, who scientifically substantiated the molecular structure of substances and gave chemistry many of the most important concepts and laws.

Mass units of elements

Initially, the hydrogen atom was taken as the basic unit of atomic and molecular mass as the lightest of the elements in the universe. But atomic masses were mostly 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 a.m.u.) is 1/12 of the mass of this standard. Currently atomic mass called the mass of an atom, which should be expressed in amu.

Mass of molecules

The mass of a molecule of any substance is equal to the sum of the masses of all the atoms that form this molecule. Hydrogen has the lightest molecular weight of a gas, its compound is written as H 2 and has a value close to two. The water molecule consists of an oxygen atom and two hydrogen atoms. Hence, its molecular weight is 15.994 + 2*1.0079=18.0152 a.m.u. The largest molecular weights have complex organic compounds- proteins and amino acids. 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.

mole

Simultaneously with the standard units of mass and volume in chemistry, a very 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 there are in 12 grams of the 12 C isotope.

When applying the 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 should be indicated exactly which structural units in question- for example, a mole of H + ions, a mole of H 2 molecules, and so on.

Molar and molecular weight

The mass of an 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 an equation

ν = k × m/M, where k is the coefficient of proportionality.

It is easy to say that for any ratios the coefficient of proportionality 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. From this we can conclude that the molar and molecular weights have the same numerical values.

Gas volumes

As you know, all the substances around us can be in solid, liquid or gaseous form. 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 final 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 distances in liquids or solids. For example, one mole of water under normal conditions occupies a volume of 18 ml - approximately the same amount fits in one tablespoon. The volume of one mole of fine crystalline table salt- 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. One mole of nitrogen under normal conditions occupies 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 ones. This is due to the difference in distances between the molecules of substances in different aggregate states.

Normal conditions

The state of any gas is highly dependent on temperature and pressure. For example, nitrogen at a temperature of 20 ° C occupies 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 at 101.3 kPa.

For normal conditions, a special abbreviation is accepted - n.o. Sometimes this designation is not written in tasks, then you should carefully reread the conditions of the problem and bring the given gas parameters to normal conditions.

Calculation of the volume of 1 mol 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) \u003d 28 g / mol.

Empirically, it was found that under normal conditions, the density of nitrogen is 1.25 g / liter.

Let's substitute this value into the standard formula known from the school physics course, where:

V is the volume of gas;
m is the mass of gas;
ρ is the gas density.

We get that the molar volume of nitrogen under normal conditions

V (N 2) \u003d 25 g / mol: 1.25 g / liter \u003d 22.4 l / mol.

It turns out that one mole of nitrogen occupies 22.4 liters.

If you perform this operation with all existing gaseous substances, you can come to a surprising 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 physico-chemical 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.

Results

The molecular weight of gaseous substances is important for determining the amount of a substance. And if the 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 satisfied:

ν = m/ M ν= V/ V m.

If we remove the constant ν, we can equate these two expressions:

So you can calculate the mass of one portion of the substance and its volume, and the molecular weight of the substance under study becomes known. By applying this formula, the volume-mass ratio can be easily calculated. When reducing this formula 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 gas under study.

It should be remembered that a strict correspondence between the real molecular weight of a substance and 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 make changes to the third or fourth digit after the decimal point in the result found. Therefore, for school tasks and experiments, the results found are quite plausible.