Twice the vapour density gives:

1. actual vapour density

2. relative vapour density

3. molecular mass

4. molar volume

molecular mass

Reason

Relative V.D. = $\dfrac{\text{Mass of vol. 'v' of the gas under similar conditions}}{\text{Mass of vol. 'v' of hydrogen gas under similar condition}}$

According to Avogadro's Law, volumes at the same temperature and pressure may be substituted by molecules. Hence,

Relative V.D. = $\dfrac{\text{mass of 1 molecule of gas or vapour}}{\text{mass of two atoms of hydrogen }}$

(Molecule of hydrogen contains 2 atoms) By multiplying both sides by 2

2 x Relative V.D = Relative molecular mass of a gas or vapour

So, twice the vapour density gives relative molecular mass.