In a molecular weight determination, a volume of an unknown gas effused through a small opening in 40 seconds while the same volume of O2 required only 20 seconds for effusion. Calculate the molecular weight of the unknown gas.

rate1/rate2 = (sqrt M2/M1)

No volume is listed for how long it took for the gas and O2 to effuse. I assume 1 L. So rate 1 = 1/40 and rate 2 = 1/20. post your work if you gt stuck.