# A sample of gas is collected over water at a temperature of 27.0°C when the barometric pressure reading is 750.0 mm Hg. What is the partial pressure of the dry gas?

The partial pressure of the collected gas is $750$ $m m$ $H g$ LESS the saturated vapour pressure of water at $27.0$ ""^@C.
The saturated vapour pressure of water at $27.0$ ""^@C ($300$ $K$) is approx. $28$ $m m$ $H g$. The actual value should of course have been quoted with the problem. Thus the partial pressure of the gas is $750.0$ $m m$ $H g$ $-$ $28$ $m m$ $H g$ $=$ $722$ $m m$ $H g$.