during an effusion experiment, oxygen gas passed through a tiny hole 2.5 times faster than the same number of moles in another gas under the same conditions. What is the molar mass of the unknown gas?

Answer 1

The easy way to do these is to assume some convenient number for the rate of effusion. The number 1 is an easy one to use.
unknown gas rate = 1L/min
oxygen gas rate = 2.5*1L/min = 2.5 L/min

(rate O2/rate unk) = swrt(MMunk/MMO2)
(2.5/1) = sqrt(MMunk/32)
Solve for molar mass (MM) unk.