Question

Vapour density of a volatile substance is$1.2$(${C_2}{H_6} = 1$).Its molecular mass would be_______
(A) $1.2$
(B) $2.4$
(C) $36$
(D) $72$

Answer 1

Vapour density is defined as the mass of the certain volume of a substance divided by mass of the same volume of hydrogen. It has no units. Vapour density is found out by dividing the molecular mass of a substance by two. The molecular mass can be found out by this equation.

Complete step-by-step answer: We know that vapour density is the molecular mass divided by two.
We know that molar mass = $2 \times$ vapour density
Molar mass of ethane = $30$ gm
Therefore, vapour density= $\dfrac{{30}}{2} = 15$
Relative vapour density is $1.2$ for ethane
Therefore Vapour density= $15 \times 1.2 = 18$
Hence the molar mass of substance $= 18 \times 2 = 36$
So the molar mass of the substance is thirty six.

Therefore the correct answer is option C.

Note: Vapour density is the ratio of mass of the volume of the gas to the mass of an equal volume of hydrogen . It is measured under the same conditions of temperature and pressure.