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How to Determine the Molar Mass of a Liquid?

Published in Chemistry Calculations 3 mins read

The molar mass of a liquid can be determined experimentally by relating its mass to the number of moles present, often using the ideal gas law after vaporizing the liquid.

Here's a breakdown of the method:

1. Vaporize a Known Mass of the Liquid:

  • Start with a small, accurately measured mass of the liquid.
  • Heat the liquid in a closed container of known volume until it completely vaporizes. Ensure the temperature is high enough to convert all the liquid into gas.

2. Measure the Temperature and Pressure of the Vapor:

  • Once the liquid has completely vaporized, carefully measure the temperature (T) and pressure (P) of the vapor inside the container. It's crucial that all the liquid has converted to gas before these measurements are taken.

3. Determine the Volume of the Vapor:

  • The volume (V) of the vapor is equal to the volume of the container it occupies.

4. Apply the Ideal Gas Law:

  • The ideal gas law is expressed as: PV = nRT, where:

    • P = Pressure (in atmospheres, atm)
    • V = Volume (in liters, L)
    • n = Number of moles
    • R = Ideal gas constant (0.0821 L·atm/mol·K)
    • T = Temperature (in Kelvin, K)

  • Rearrange the ideal gas law to solve for the number of moles (n):

    n = PV / RT

5. Calculate the Molar Mass:

  • Once you've determined the number of moles (n) and you know the mass (m) of the original liquid, you can calculate the molar mass (M) using the following formula:

    Molar Mass (M) = mass (m) / number of moles (n)

Example:

Let's say you vaporize 0.100 grams of a liquid in a 250 mL (0.250 L) flask. After vaporization, the temperature is 100°C (373 K) and the pressure is 750 mmHg (0.987 atm).

  1. Calculate the number of moles (n):
    n = (0.987 atm 0.250 L) / (0.0821 L·atm/mol·K 373 K)
    n = 0.00807 moles

  2. Calculate the molar mass (M):
    M = 0.100 g / 0.00807 mol
    M = 12.4 g/mol

Therefore, the molar mass of the liquid is approximately 12.4 g/mol.

Important Considerations:

  • Ensure accurate measurements of mass, temperature, pressure, and volume for best results.
  • This method assumes ideal gas behavior. Deviations from ideal behavior may occur at high pressures or low temperatures, potentially affecting the accuracy of the result.
  • The liquid must be completely vaporized.
  • The liquid should be relatively pure. Impurities can affect the vapor pressure and molar mass calculation.

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