Question

The vapour density of gas A is three times that of gas B . If the molar mass of A Is M ,then molar mass of B is

Answer 1

vd of A is x
vd of B is 3x
molar mass of A is M=2x
molar mass of B is 6x
2x=M
x=M/2
6x=3M

Answer 2

By Avogadro’s Hypothesis, molar mass of a gas is double the vapour density of it. According to the given problem, let the first gas have a vapour density of V and therefore, the second gas has a vapour density of 3V (three times).  

The molar mass (M) of the first gas is twice that of its vapour density = 2V and the second gas has a molar mass of 6V.

Now,  

Molar mass of gas A is M = 2V, therefore V = M/2

So, molar mass of gas B is 6V = 3M