Education.com
Try
Brainzy
Try
Plus

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:

Synopsis

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.

Equipment

      analytical balance
      barometer
      beaker(s)
      clamp
      Erlenmeyer flask
      graduated cylinder
      support stand

Measurements

  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

Calculations

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.

Comments

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

Add your own comment

Ask a Question

Have questions about this article or topic? Ask
Ask
150 Characters allowed