Question

The density of a mixture of $N_2$ and $O_2$ at NTP is $1.3g{L}^{-1}$. Calculate partial pressure of $O_2$.

Answer 1

Let $n_1$ and $n_2$ moles of $N_2$ and $O_2$ be present in the mixture respectively.
Average molecular mass of the mixture $=\frac{28n_1+32n_2}{n_1+n_2}..\left(i\right)$
Average molecular mass from general gas equation,
$M=\frac{D}{P}\times RT=\frac{1.3}{1}\times 0.0821\times 273=29.137..\left(ii\right)$
So, $\frac{28n_1+32n_2}{n_1+n_2}=29.137$ or $\frac{4n_2}{n_1+n_2}=1.137$
or $\frac{n_2}{n_1+n_2}=0.28=$ mole fraction of oxygen
Partial pressure of $O_2=0.28\times 1=0.28atm$.