Posted by Peter on Tuesday, April 15, 2014 at 10:21pm.
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?

Chemistry  DrBob222, Tuesday, April 15, 2014 at 11:21pm
(rate1/rate2) = sqrt(M2/M1)
Substitute and solve for molar mass. 
Chemistry  Peter, Tuesday, April 15, 2014 at 11:30pm
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 
Chemistry  DrBob222, Tuesday, April 15, 2014 at 11:48pm
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).