Experiment 5: Molar Volume of a Gas for AP Chemistry

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

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


The volume occupied by a mole of a gas is calculated in this experiment. A sample of a solid substance is heated, decomposing it into several products, including a gas. The mass of the gas is determined by the weight difference of the solid before and after heating and is then converted to moles. The volume of the gas, the pressure, and temperature are measured.


      analytical balance
      Erlenmeyer flask
      graduated cylinder
      rubber tubing
      test tube(s)


  1. the barometric pressure
  2. the mass of the test tube plus solid sample (before the reaction)
  3. the mass of the test tube plus sample after the reaction
  4. the temperature of the water
  5. volume of water displaced into the beaker, if the variation is used

The mass of the test tube after the reaction must be determined after the test tube has completely cooled to room temperature.


The temperature must be converted to kelvin (T1), and the volume of water is normally expressed in liters.

The mass of gas generated is calculated by taking the difference between measurements 2 and 3. Using the molar mass of the gas, the mass of gas is converted to moles of gas (n).

For the variation, the vapor pressure of water at the recorded temperature is found in a table.

The pressure of the gas (P1) is the difference between the value in the table and measurement 1.

The volume of water in the beaker is the volume of the gas (V1).

Calculate the volume (V2) of the gas at STP (T2 and P2) using the combined gas law.

The molar volume of the gas is the volume at STP (V2) divided by the moles of gas (n).


The most common procedure is to produce oxygen gas by decomposing KClO3. A common variation is to measure the volume of gas produced by displacing water from a flask. The volume of water displaced is the volume of gas generated at that temperature and pressure. From a measurement of the atmospheric pressure and the temperature of the gas, the volume of gas at STP can be calculated.

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