Molecular mass is the weight of all atoms in the corresponding molecule. This indicator is expressed in amu (atomic units of mass).
This chemical quantity is sometimes calculated in kilograms. To do this, it is multiplied by 1.66057 * 10
-27 . This number is a mass of 1 amu and corresponds to 1/12 of the weight of one atom of the nuclide
12 C. Most often they use the
relative molecular mass, which is the ratio of the weight of the molecule to the mass of one atomic unit.
It is worth noting that this indicator helps to characterize the mass of the molecule, taking into account all the isotopes that form the corresponding chemical compound.
In a macroscopic unit of quantity of a substance called a "mole", there is a constant number of any particles - atoms or molecules, electrons or pairs of ions. This amount is 6.022 * 10 23 (the so-called Avogadro constant). To get the relative molecular weight, you should multiply the weight of the molecules by their number and divide the result by the product of the Avogadro constant and the mass of one atomic unit.
How is the molecular weight of a substance determined?
The easiest way to determine this parameter is to use a special formula according to which it is necessary to divide the weight of the compound by its amount.
If the chemical formula of the compound is known, the molecular weight is easily determined using the periodic table, since this value is indicated for each element in it and corresponds to its serial number. It is enough to find the sum of the molecular weights of all the components of the compound.
According to Avogadro’s law, gases of the same volume contain the same number of molecular particles. Given this pattern, the Mendeleev-Klaiperon equation was derived . The molecular weight of the gas can be determined using the appropriate formula.
To carry out the calculations, it is necessary to know a lot of indicators - the weight of the substance, the universal gas constant, the temperature in Kelvins, as well as the pressure in Pascals and the volume in m
3 . Due to this amount of data, the errors in the calculations are minimal, but it should be noted that the Mendeleev-Klaiperon equation is valid only for gaseous substances.
The molecular weights of volatile compounds can be determined by mass spectrometry or gas chromatography using a Martin gas balance. The average value of this indicator for polymeric substances with a high degree of polymerization is calculated taking into account their rheological characteristics, as well as the properties of their solutions (for example, viscosity or the ability to scatter light).
Why is molecular weight determined?
The numerical value of this value is used to identify a variety of chemical compounds, as well as to identify individual nucleotides in the test substance. In addition, this chemical indicator is often determined in the study and synthesis of high molecular weight compounds whose properties depend on this particular parameter.