A sample of atmospheric air contains nitrogen and oxygen, and is saturated with water vapour under a — States of Matter and Gaseous State Chemistry Question
Question
A sample of atmospheric air contains nitrogen and oxygen, and is saturated with water vapour under a total pressure of $640\text{ mm Hg}$. If the vapour pressure of water (aqueous tension) at that temperature is $40\text{ mm Hg}$ and the molar ratio of $N_2 : O_2$ is exactly $3:1$, what is the partial pressure of nitrogen in the sample?
Answer: B
💡 Solution & Explanation
The pressure of the dry gas mixture ($N_2 + O_2$) is $P_{dry} = 640 - 40 = 600\text{ mm Hg}$. Since the molar ratio of $N_2:O_2$ is $3:1$, the mole fraction of $N_2$ in the dry gas is $3/(3+1) = 3/4$. Therefore, the partial pressure of $N_2$ is $(3/4) \times 600\text{ mm Hg} = 450\text{ mm Hg}$.
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