A gas collected over water at 1 atm has water vapour pressure 17.5 mmHg. The pressure of the dry gas is found by:

1. Gas collected over water is a mixture of the gas and water vapour. 2. Dalton's law of partial pressures gives $P_{total}=P_{gas}+P_{water}$. 3. Rearranging, $P_{gas}=P_{total}-P_{water}$. 4. With $P_{total}=760$ mmHg and $P_{water}=17.5$ mmHg, the dry gas is at $742.5$ mmHg. 5. That corrected pressure then goes into $m=\dfrac{PVM_r}{RT}$; using the uncorrected total overstates the mass. _Source: NECTA ACSEE 2023 Chemistry 132/1, Question 1: Gases_