Vapour density of a gas $A$ is three times that of gas $B$. If the molecular mass of $B$ is $X$, the molecular mass of $A$ is:

The correct option is B $3X$

Vapour density of a gas $A$ is three times that of gas $B$. If the molecular mass of $B$ is $X$, the molecular mass of $A$ is $3X$.

Note: The molecular mass is twice the vapour density. Hence, the ratio of molecular masses is equal to the ratio of vapour densities.

If the vapour density of a gas $A$ is three times that of gas $B$, then the molecular mass of gas $A$ is also three times that of gas $B$.