This experiment, designed for a general chemistry laboratory, illustrates the use of Dalton's law of partial pressures to determine the vapor pressure of a volatile liquid. A predetermined volume of air is injected into a calibrated tube filled with a liquid whose vapor pressure is to be measured. The volume of the liquid displaced is greater than the volume of air injected because of the vapor pressure of the liquid. Using Dalton's law of partial pressures, the sum of the partial pressure of the liquid and the air in the tube is directly related to the final volume, allowing the calculation of the vapor pressure of the liquid. (Contains 2 notes.)