CHEMISTRY · Single correct · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A student collects O₂ over water at 22 °C. The total pressure in the collection bottle is 765 torr, and the vapor pressure of water at 22 °C is 20 torr. Which of the following best explains why the student must subtract 20 torr when calculating the amount of dry O₂?
(a) Because, according to Dalton’s law, the combined partial pressures should equal 760torr at sea level; subtracting 20torr prevents the calculated pressure from exceeding this limit.
(b) Because water vapor molecules are heavier than O2 molecules and therefore contribute less pressure per mole, so their pressure must be subtracted to correct for the difference.
(c) Because part of the observed pressure is due to water molecules that occupy space and contribute to the total pressure.
(d) Because some of the pressure reading is caused by the height of the water column above the gas, so 20torr is subtracted to account for that hydrostatic pressure.