What is the molecular mass of this gas?

Question

Oxygen gas i sconfined in a chmaber under water and bubbles out through a tiny orifice. It takes 62.5 seconds for the 20.0 mL of oxygen to effuse out of the chamber. Another gas of unknown molecular weight takes 35.5 seconds for 15.0 mL to effuse out of the same chamber at the same temperature and pressure. What is the molecular mass of this gas?

Answer 1

r1/r2=Sqrt[M2/M1]

oxygen has M=32g/mol r=15ml/46.875seconds= .32ml/sec
uknown has M=? r=15ml/35.5seconds= .4225ml/sec

.32/.4225=Sqrt[M/32] m=18.36 g/mol

this is very close to water's molar mass