Question

A sample of O2 gas is observed to effuse through a porous barrier in 5.69 minutes. Under
the same conditions, the same number of moles of an unknown gas requires 11.7 minutes
to effuse through the same barrier. What is the molar mass of the unknown gas?
a) 11.6 g/mol
b) 135 g/mol
c) 7.56 g/mol
d) 66.5 g/mol
e) More information is needed

Answer 1

Answer:

MX = 135 g/mol

Explanation:

The rate of effusion (r) is the distance travelled (d) over the time elapsed (t). Let's call the uknown gas X. The ratio of rates rO₂/rX is:

$\frac{rO_{2}}{rX} =\frac{\frac{d}{5.69min} }{\frac{d}{11.7min} } =2.06$

We can find the molar mass of X using Graham's law.

$\frac{rO_{2}}{rX}=\sqrt{\frac{MX}{MO_{2}} } \\2.06=\sqrt{\frac{MX}{32.00g/mol} }\\ MX=135g/mol$