Helium diffuses 4 times faster than an unknown gas under similar conditions. Find the vapour density of unknown gas.

Explanation

Ratio of rates of diffusion of helium and unknown gas is given as
` (r_1)/(r_2) = sqrt((M_2)/(M_1)) implies 4/1 = sqrt((M_2)/(4))`
`M_2 = 4 xx 4^2 = 64`
Molecular weight of unknown gas = 64
Vapour density of unknown gas =32 .