[sundrops]

1.894×10-4 mol of an unidentified gaseous substance effuses through a tiny hole in 89.2 s.
Under identical conditions, 1.560×10-4 mol of argon gas takes 82.1 s to effuse.
What is the molar mass of the unidentified substance (in g/mol)?

I don't even know how to approach this problem. Anyone have any ideas on where to start?

I tried going mol/s for the rate of effusion - but that doesn't help me very much... please *delete me*

[sundrops]

here's exactly what I tried:

rate of argon:
(1.560E-4 / 82.1s) = 1.900122E-6 mol/s

rate of unknown:
(1.894E-4mol / 89.2s) = 2.12332E-6mol/s

rate argon / rate unkniwn = {square root(molar mass unknown) / square root(molar mass argon)}

so:
 (1.900122E-6 mol/s)(square root(39.948g/mol)) / (2.12332E-6mol/s) = square root(molar mass unknown)

which gives me the answer of 2.378 g/mol of the unknown


however the answer is wrong and I've only got 2 tries left!  

[Donaldson Tan]

see attached

[Donaldson Tan]

i copied one of my numerical values wrongly into the final equation.

1.10865 = sqrt(M/39) / 0.030429

so M = 37.27g/mol