It took 20 seconds for a certain

volume of oxygen to effuse
through an orifice(small opening) at
298K. How long will it take the
same volume of each of the
following gases to effuse through the same orifice at the same
temperature?. The gases are
hydrogen, helium, and chlorine.
(H=1, He=4, O=16 and Cl=35.5).

1 answer

(rate 1/rate 2) = sqrt(M2/M1) where M2 and M1 are molar masses.
Make up a number for the "certain volume". I would use a convenient number like 100 L. Then
rate1 for oxygen = 100 L/20 = 5 L/s
rate 2 = ?
M2 = 2 for H
M1 = 32 for O
Substitute into the above and solve for rate 2. Then rate 2 = 100/seconds. You know rate 2 and 100L, solve for seconds.
The others are done the same way. Post your work if you get stuck.