Explanation of the Vapour Density of Hg Vapour
Vapour density is defined as the ratio of the mass of a certain volume of a gas to the mass of an equal volume of hydrogen gas at the same temperature and pressure. In the case of mercury (Hg) vapour, its vapour density is 6.92 with respect to air.

Calculating the Molecular Mass of Hg
To determine the molecular mass of mercury, we need to first calculate the molar mass of the gas. Given that the vapour density of Hg vapour is 6.92, we know that the molar mass of mercury vapour is 6.92 times that of hydrogen gas (H2), which has a molar mass of approximately 2 g/mol.
Therefore, the molar mass of mercury vapour is 6.92 x 2 = 13.84 g/mol.

Converting Molar Mass to Molecular Mass
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule. The atomic mass of mercury (Hg) is approximately 200.59 g/mol.
Since the molar mass of mercury vapour is 13.84 g/mol, we can calculate the molecular mass of mercury vapour by multiplying the molar mass by the atomic mass of mercury:
Molecular mass = Molar mass x Atomic mass
Molecular mass = 13.84 g/mol x 200.59 g/mol
Molecular mass ≈ 200.68 g/mol
Therefore, the molecular mass of mercury vapour is approximately 200.68 g/mol.