# A 250 mL sample of oxygen is collected over water at 25°C and 760.0 torr pressure. What is the pressure of the dry gas alone?

Apr 29, 2016

You need to know the $\text{saturated vapour pressure}$ of water at $25$ ""^@C.

#### Explanation:

The gas collected over water consists of (i) whatever the gas is, and (ii) the saturated vapour pressure. At $25$ ""^@C, $\text{SVP}$ $=$ $23.8$ $m m \cdot H g$. At higher temperatures, how would you expect $\text{SVP}$ to evolve.

Thus the pressure exerted by the gas is thus $\left(760 - 23.8\right) \cdot m m \cdot H g$.

$\text{SVP}$ would always be given in such a problem.