Experiment 3: Molar Mass by Vapor Density for AP Chemistry

By — McGraw-Hill Professional
Updated on Feb 9, 2011

For a quick review on gas law relationships, refer to the following concepts:


The molar mass (molecular mass) of a volatile substance is determined in this experiment. The mass of a sample of vapor is initially determined. This mass, along with the volume of the container, the pressure, and the temperature, is used with the ideal gas equation to calculate the molar mass.


      analytical balance
      Erlenmeyer flask
      graduated cylinder
      support stand


  1. the mass of the flask
  2. the mass of the flask plus condensed vapor
  3. the temperature of the water bath used to heat the flask
  4. the barometric pressure
  5. the number of milliliters of water required to fill the flask


There are a variety of ways to do the calculations. Most of these, however, involve the calculation of the number of moles (n) from the ideal gas equation: n = PV /RT. The mass of the vapor sample is calculated from the difference between measurements 1 and 2. The temperature (measurement 3) is converted to kelvin. The pressure (measurement 4) is converted to atmospheres. Measurement 5 is converted to liters. Inserting the various numbers into the ideal gas equation allows you to calculate the number of moles. The molar mass is calculated by dividing the mass of the sample by the moles.


Variations in this experiment usually combine the ideal gas equation with the mass of the sample.

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