how would you set this problem up?

An unknown gas effusees at .850 times the effusion rate of nitrogen dioxide, NO2. Estimate the molar mass of the unknown gas.

1 answer

rate1/rate2 = sqrt[M2/M2]
I would arbitrarily pick a number, say 10 cc/min, for the effusion rate of NO2. Then 850*10 = the rate for the unknown gas. Substitute into the above equation and solve for M2.