Molecular mass is expressed as the sum of the masses of the atoms that make up the molecule of a substance. Usually it is expressed in a.u.m., (atomic mass units), sometimes also called d alton and denoted by D. For 1 a.m.u. today, 1/12 of the mass of C12 of a carbon atom is accepted, which in units of mass is 1, 66057.10-27 kg.
Thus, the atomic mass of hydrogen equal to 1 shows that the hydrogen atom H1 is 12 times lighter than the carbon atom C12. Multiplying the molecular weight of a chemical compound by 1, 66057.10-27, we get the value of the mass of the molecule in kilograms.
In practice, however, they use a more convenient value Mot=M/D, where M is the mass of the molecule in the same mass units as D. The molecular mass of oxygen, expressed in carbon units, is 16 x 2=32 (the oxygen molecule is diatomic). In the same way, in chemical calculations, the molecular weights of other compounds are also calculated. The molecular weight of hydrogen, in which the molecule is also diatomic, is, respectively, 2 x 1=2.
Molecular weight is a characteristic of the average mass of a molecule, it takes into account the isotopic composition of all elements that form a given chemical substance. This indicator can also be determined for a mixture of several substances, the composition of which is known. In particular, the molecular weight of air can be taken as 29.
Earlier in chemistry, the concept of a gram-molecule was used. Today, this concept has been replaced by a mole - the amount of a substance containing the number of particles (molecules, atoms, ions) equal to the Avogadro constant (6.022 x 1023). Until today, the term "molar (molecular) weight" is also traditionally used. But, unlike weight, which depends on geographic coordinates, mass is a constant parameter, so it’s still more correct to use this concept.
The molecular weight of air, like other gases, can be found using Avogadro's law. This law states that under the same conditions in the same volumes of gases there are the same number of molecules. As a result, at a certain temperature and pressure, a mole of gas will occupy the same volume. Considering that this law is strictly observed for ideal gases, a mole of a gas containing 6.022 x 1023 molecules occupies at 0 ° C and a pressure of 1 atmosphere a volume equal to 22.414 liters.
The molecular weight of air or any other gaseous substances is as follows. The mass of some known volume of gas is determined at certainpressure and temperature. Then, corrections are introduced for the non-ideality of the real gas, and using the Clapeyron equation PV=RT, the volume is reduced to pressure conditions of 1 atmosphere and 0 ° C. Further, knowing the volume and mass under these conditions for an ideal gas, it is easy to calculate the mass of 22.414 liters of the investigated gaseous substance, that is, its molecular weight. This is how the molecular weight of air was determined.
This method gives sufficiently accurate values of molecular weights, which are sometimes used even to determine the atomic weights of chemical compounds. For a rough estimate of molecular weight, the gas is usually assumed to be ideal, and no additional corrections are made.
The above method is often used to determine the molecular weights of volatile liquids.
