Molecular weight is the weight of all the atoms that make up the corresponding molecule. This indicator is expressed in amu. (atomic mass units).
This chemical quantity is sometimes calculated in kilograms. To do this, it is multiplied by 1, 6605710-27. This number is the mass of 1 amu. and corresponds to 1/12 of the weight of one atom of the nuclide 12С. The most commonly used is the relative molecular weight, which is the ratio of the weight of a molecule to the mass of one atomic unit.
It is worth noting that this indicator helps to characterize the mass of a molecule, taking into account all the isotopes that form the corresponding chemical compound.
In the macroscopic unit of the amount of matter, which is called "mole", there is a constant number of any particles - atoms or molecules, electrons or pairs of ions. This number is 6.0221023(the so-called Avogadro constant). To get the relative molecular mass, 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 atomicunits.
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 quantity.
If the chemical formula of a compound is known, the molecular weight is easily determined using the Periodic Table, since in it this value is indicated for each element and corresponds to its serial number. It is enough to find the sum of the molecular weights of all the constituents of the compound.
According to Avogadro's law, gases of the same volume contain the same number of molecular particles. Given this regularity, the Mendeleev-Klaiperon equation was derived. The molecular weight of a gas can be determined using the appropriate formula.
To carry out calculations, you need to know quite a lot of indicators - the weight of a substance, the universal gas constant, temperature in Kelvin, as well as pressure in Pascals and volume in m3. Due to this amount of data, the errors in the calculations are minimal, but it should be noted that the Mendeleev-Claiperon equation is valid only for gaseous substances.
Molecular weights of volatile compounds can be determined by mass spectrometry or gas chromatography using Martin gas balances. 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 asproperties of their solutions (for example, viscosity or ability to scatter light).
Why is molecular weight determined?
The numerical value of this value is used to identify a wide 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 macromolecular compounds, the properties of which depend precisely on this parameter.
