I don't understand this chem homework

Question

If the vapor pressure of water at 2.8 degrees C is 11.574 mm of Hg, and the pressure of a gas collected over water is 781.2 torr, what is the pressure of the gas only (dry)?

Answer 1

When you collect a gas over water, as in this case, you MUST subtract from the total pressure (gas + water vapor), the pressure due to the water vapor in order to find the pressure of the gas alone. Put another way

Total pressure = pressure of the gas + vapor pressure of water

Pressure of the dry gas = 781.2 torr - 11.574 torr = 769.6 torr