Under identical conditions separate samples of O2 and an unk

Under identical conditions, separate samples of O2 and an unknown gas were allowed to effuse through identical membranes simultaneously. After a certain amount of time, it was found that 5.79 mL of O2 had passed through the membrane, but only 3.58 mL of of the unknown gas had passed through. What is the molar mass of the unknown gas?

Solution

rate of effusion is inversely propional to square root of molar mass
if M is molar mass of unknown gas
and molar mass of O2 = 32 g/mol

use,
(volume of O2)/(volume of unknown gas) = (M/32)^1/2
5.79/3.58 = (M/32)^1/2
1.62 = (M/32)^1/2
(M/32) = 2.62
M = 84.0 g/mol

Answer : 84.0 g/mol