Chemistry
posted by Camarra .
A sample of hydrogen gas (H2) was found to effuse at a rate equal to 6.7 times that of an unknown gas. The molecular weight of the unknown gas is __________ g/mol. Express your answer to one (tenth) decimal place.

I would make up a rate for the unknown gas and multiply that made up rate x 6.7 to find the rate of H2. Plug in those numbers.
rate1/rate2 = sqrt(M2/M1)
Solve for M (depending upon what you call rate 1 and rate2, M2 and M1.