A sample of nitrogen gas is collected over water at a temperature of 23.0°C. What is the pressure of nitrogen gas if atmospheric pressure is 785 mm Hg?

Aug 2, 2016

The saturated vapour pressure of water at $23$ ""^@C is 21.1 mm Hg.

Explanation:

Gas collected over water must always account for the saturated vapour pressure. I have included a link to a Table, but in an exam these data would be supplied.

Thus $\text{atmospheric pressure}$ $=$ ${P}_{\text{SVP" + P_"dinitrogen}}$

${P}_{\text{dinitrogen}} = \left(785 - 21.1\right) \cdot m m \cdot H g$ $=$ $763.8 \cdot m m \cdot H g$.

Note that when a gas is collecgted over water, we must always include ${P}_{\text{SVP}}$.