Molecular mass from vapour density
The density of a gas relative to that of hydrogen at the same temperature and pressure is known as the vapour density or the hydrogen density of the gas.
Thus, the vapour density of a gas =
∴ the relative molecular mass of a gas = 2 x vapour density.
The vapour densities of gases can be determined experimentally. So can those of many other substance like methanol, ethanol and acetone, which can be completely volatilized. Thus, it is easy to calculate the relative molecular masses of such substances.
|Substance||Vapour density||Relative molecular mass (= 2 x vapour density)||Molecular formula|
The atomicity of a gaseous element can be easily calculated if we know its vapour density and atomic mass.
Atomicity = .
For example, the vapour density of nitrogen is 14 and so its molecular mass = 2 x 14=28
As the atomic mass of nitrogen is 14, its atomicity = .
Hence the molecular formula N2.
The percentage composition of a compound in terms of the different elements constituting it can be easily calculated if we know the molecular or the empirical formula of the compound and the atomic masses of its constituents.
Illustration 6: What is the percentage of Ca is CaCO3? (Ca = 40, C = 12, O =16)
Solution: The relative molecular mass of CaCO3 = 40 + 12 + 3 x 16 = 100.
100 g of CaCO3 contain 40 g of Ca.
∴ the percentage of Ca in CaCO3 = 40.
Exercise 10: Calculate the percentages of H and O in H2O. (H = 1, O = 16)
Ans. 11.11 % of H, 88.89 % of O.
Exercise 11: Calculate the percentage of Cu and water of crystallisation in blue vitriol CuSO4 . 5H2O).
(Cu = 63.5, S = 32, O = 16, H = 1)
Ans. 25.45 % of Cu, 76.07 % of H2O
Exercise 12: Calculate the percentage composition of ethanol (C2H5OH) in terms of all the elements present in the compound.
Ans. 52.17 % of C, 13.04 % of H, 34.78 % of O