Calculate the relative molecular mass and vapour density of methyl alcohol [CH₃OH] if 160 g. of the alcohol on vaporization has a volume of 112 litres at s.t.p.

Given,

Weight of 112 lit of CH₃OH = 160 g [at s.t.p.]

∴ Weight of 1 lit of CH₃OH = $\dfrac{160}{112}$ = 1.4286 g

Weight of 1 lit of H₂ = 0.09 g [at s.t.p.]

Under similar temperature and pressure,

Vapour density of gas methyl alcohol =

$\dfrac{\text{Wt. of certain vol. of gas}}{\text {Wt. of same vol. of H}_2} \\[0.5em] = \dfrac{1.4286}{0.09} \\[0.5em] = 15.87 \approx 16$

Molecular weight = 2 x Vapour density
= 2 x 15.87 = 31.74 g ≈ 32 g

Hence, molecular weight of methyl alcohol is 32 g and vapour density of methyl alcohol is 16