Question

A sample of air contains only $N_{2},O_{2}$ and $H_{2}O$ . It is saturated with water vapours and the total pressure is 640 torr. The vapour pressure of water is 40 torr and the molar ratio of $N_{2}:O_{2}$ is 3 : 1. The partial pressure of $N_{2}$ in the sample is

• A. $\text{480 torr}$
• B. $\text{600 torr}$
• C. $\text{525 torr}$
• D. $\text{450 torr}$

Answer 1

Answer: (D)

Pressure of dry air $=P_{m o i s t \, a i r}-P_{w a t e r \, v a p o u r s}=640-40=600torr.$
Partial pressure = mole fraction $\times P_{T}$
$\therefore $ Partial pressure of $N_{2}$ and $O_{2}$ are also in the ratio of 3 : 1.
Partial pressure of $N_{2}=\frac{600 \times 3}{3 + 1}=450 \, torr$ .