How to Find Molar Mass of Vapor?

The molar mass of a vapor can be determined by a specific experimental procedure involving condensing the vapor back into a liquid. This process, as outlined in the reference, relies on precise measurements of mass.

Determining Molar Mass of a Vapor Experimentally

Here's how you can experimentally determine the molar mass of a condensable vapor:

• Preparation:

  1. Start with a clean, dry, and weighed bulb. Note this initial mass.
  2. Add an excess amount of the corresponding liquid to the weighed bulb.

• Vaporization:

  1. Heat the assembly (bulb with liquid) carefully.
  2. This heat volatilizes all the liquid, turning it into vapor.
  3. Continue heating to expel all excess vapor, leaving only saturated vapor in the bulb at the heating temperature and external pressure.

• Condensation and Measurement:

  1. Cool the bulb containing only the vapor, allowing it to condense back into a liquid.
  2. Carefully weigh the bulb again with the condensed vapor. Note this mass.
  3. Calculate the mass of the vapor using the initial and final weights of the bulb.

• Calculation:

  1. Using the experimental parameters (mass of vapor, temperature, volume, and external pressure) and the ideal gas law (PV=nRT), determine the number of moles of the vapor (n).
  2. The molar mass (M) of the vapor is then calculated using the mass of the vapor (m) and the number of moles using the formula M= m/n.

Summary of the Process

This method, using the process of condensation to find the mass of the vapor, is a reliable technique for determining the molar mass of many condensable vapors, by calculating the number of moles, which allows you to find the molar mass.