Chemistry
posted by Peter .
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 4.92 mL of O2 had passed through the membrane, but only 3.04 mL of of the unknown gas had passed through. What is the molar mass of the unknown gas?

(rate1/rate2) = sqrt(M2/M1)
Substitute and solve for molar mass. 
I got to 1.62 = sqrt(M unknown/M O2)
I know that the molar mass of O2 is 32
So I squared both sides of the equation to get 2.6244 = M unknown/ 32g O2
Multiplied 2.6244 x 32 = 83.98g/molunknown 
I differ slightly (84.82) probably because you rounded to 1.62 while I kept everything in the calculator until the final answer. But to three significant figures I would round the answer to 83.8 (probably krypton).