Instruction

1

Knowing the formula

**of the gas**, its molar**mass**can be calculated by elementary counting. Take carbon dioxide. Its formula is CO2. So its molar mass is: 12 + 32 (molar mass of oxygen with an index "2") = 44.2

Well, if you need to calculate the molar

**mass of**an unknown**gas**, which in some confined space, for example, a hermetically sealed container? Here we come to the aid of the universal equation Mendeleev–Clapeyron describing the state of an "ideal**gas**". Of course, none of the gas does not satisfy the conditions "perfect", but when pressure and temperature are insignificant different from normal, this equation is very convenient for calculations. And obtained when computing the error is very small and it can be safely ignored.3

The generic equation has the following form: PV = MRT/m, hder – pressure

V is the volume in cubic metres;

M – the actual weight of

m is its molar mass;

R – universal gas constant;

T – temperature

**gas**in Pascals;V is the volume in cubic metres;

M – the actual weight of

**gas**;m is its molar mass;

R – universal gas constant;

T – temperature

**of gas**in degrees Kelvin.4

You will see that the molar mass is calculated by the formula MRT/PV. For example, it is necessary to find the molar

**mass****of gas**, if you know that 3 pounds of this**gas**are in a sealed container with a volume of 1.7 cubic meter at a pressure of 100000 PA and a temperature of 27 degrees Celsius.5

Substitute the known values in this formula, of course, not forgetting first to convert to one system of quantities. Otherwise, it's complete nonsense. 3,0*8,31*300/170000=0,04399 kg/mol.

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Well, since the molar mass of the substance measured in grams per mole, multiply the result by 1000 and get the answer: molar mass

**of gas**, under such conditions, equal 43.99 grams/mol or subject to rounding - 44 grams/mol. That is, it is the same carbon dioxide.# Advice 2: How to find the molar mass of gas

In the solution of chemical problems often need to know the molar

**mass****of the gas**.To determine molar masses from chemists there are different ways – from relatively simple that can be performed in the laboratory (for example, a method of pumping**gas**and using the equation Mendeleev-Clapeyron), to complex and require specialized scientific equipment. In addition, however, to determine the molar**mass of a****gas**using a conventional periodic table.You will need

- - the periodic table,
- calculator.

Instruction

1

By definition, the molar mass is the mass of 1 mole of a substance, i.e., one portion of a substance containing 6*10 to the 23rd degree of particles (Avogadro's number). Particles can be atoms, ions or molecules. Molar mass is equal in magnitude to the relative atomic mass of the substance (or its relative molecular mass if the substance has a molecular structure).

2

Thus, to know the molar

**mass****of the gas**that has atomic structure, it is sufficient to find in the periodic table the relative atomic**mass**, which is always indicated in the table cell next to the name of the element and rounding it to an integer value. For example, for oxygen On the value of the relative atomic mass of the cell is 15,9994, rounding, will receive a 16 - therefore, the molar mass of oxygen is 16 g/mol.3

Consider the case when you want to find the molar

To do this, determine the chemical formula

**mass of a****gas**that have a more complex molecular structure.To do this, determine the chemical formula

**of gas**, which atoms are part of it. For example, the molecule carbon dioxide**gas**CO2 according to the formula contains one atom of carbon C and two atoms of oxygen O.4

Write from the periodic table relative atomic masses of all the chemical elements are included in the formula, and rounded to their nearest integer.In this example, with carbon dioxide has already been found for oxygen On the rounded value is 16; similarly, in the table to find relative atomic

**mass**of carbon equal to 12,011, and rounding it to an integer, we get 12.5

Fold now all rounded values of relative atomic masses of elements based on their quantitative ratios in the formula. For carbon dioxide

**gas**it will be:12(one carbon) + 2*16(two atoms of oxygen) = 44Вы will receive a number that represents the relative molecular**mass**of a substance is numerically equal to the molar mass - this is is the desired value of the molar mass of**gas**, we need only substitute the correct dimension.Thus, in this example the molar mass of carbon dioxide**gas**was 44 g/mol.Note

Molar mass and molecular mass are not identical to each other - despite the fact that numerically they are equal, they differ in the dimension.

The dimension of molar mass is always specified in g/mol, while the molecular mass is the mass of a molecule expressed in atomic mass units (a.e.m.).

The dimension of molar mass is always specified in g/mol, while the molecular mass is the mass of a molecule expressed in atomic mass units (a.e.m.).

Useful advice

You can help a special program to calculate molar mass you can find programs that require downloading and installation on your computer, and the online version.

# Advice 3: How to determine the molar mass of the substance

In order to find the molar

**mass****of the substance**, determine its chemical formula and using the periodic table calculate its molecular**mass**. It is numerically equal to molar mass**of the substance**in grams per mole. If you know the mass of one molecule**of the substance**, put it in grams and multiply by 6,022•10^23 (Avogadro's number). Molar**mass**of gas can be found using the ideal gas equation of state.You will need

- periodic table, pressure gauge, thermometer, scales.

Instruction

1

Determination of the molar mass

**of a substance**from the chemical formula. Locate elements in the periodic table of Mendeleev, which correspond to atoms from which a molecule**of the substance**. If a molecule**of a substance is**monatomic, then it will be its molar mass. If not, find the atomic**mass**of each element and add the masses. The result will be the molar mass**of a substance**expressed in grams per mole.2

Determination of the molar mass

**of the substance**mass of a single molecule. In that case, if we know the mass of one molecule, put it into grams, then multiply by the number of molecules in one mole of any**substance**, which is 6,022•10^23 (Avogadro's number). Get the molar**mass****of the substance**in grams per mole.3

Determination of the molar mass of the gas. Take a container which can be hermetically sealed with a predetermined volume, which translate to cubic meters. With a pump siphon his gas, on the scales and weigh the empty cylinder. Then fill it up with gas, the molar mass is measured. Again weigh the container. The difference in the mass blank and the injected gas cylinder will be equal to the mass of gas, to Express it in grams.

Using the manometer, measure the pressure of the gas inside the cylinder, to do this, connect it to the hole for gas injection. You can use cylinder with built-in pressure gauge to quickly monitor pressure indicators. Measure pressure in Pascals.

Using the manometer, measure the pressure of the gas inside the cylinder, to do this, connect it to the hole for gas injection. You can use cylinder with built-in pressure gauge to quickly monitor pressure indicators. Measure pressure in Pascals.

4

Wait some time for the gas temperature inside the container equal to the ambient temperature and measure it with a thermometer. Temperature from degrees Celsius translate to the Kelvin, which add to the measured value of the number 273.

The mass of gas multiply by the temperature and universal gas constant (8,31). The resulting number is consistently split on the values of pressure and volume (M=m•8,31•T/(P•V)). The result will be the molar mass of the gas in grams per mole.

The mass of gas multiply by the temperature and universal gas constant (8,31). The resulting number is consistently split on the values of pressure and volume (M=m•8,31•T/(P•V)). The result will be the molar mass of the gas in grams per mole.