Determination of Molar Mass in Volatile Liquids through Ideal Gas Law: Experimental Insights and Error Analysis

Categories: Chemistry

To determine the molar mass of a volatile liquid, you can use the ideal gas law and the concept of vapor density. Here are the steps:

Apparatus and Materials:

  1. Volatile Liquid: The substance whose molar mass you want to determine.
  2. Boiling Flask: To contain the liquid.
  3. Thermometer: To measure the temperature.
  4. Pressure Gauge or Manometer: To measure the pressure.
  5. Gas Collection Apparatus: To collect the gas produced.
  6. Balance: For weighing.


  1. Set up the apparatus:
    • Attach the boiling flask to the gas collection apparatus.

    • Fill the boiling flask with the volatile liquid.
    • Connect the flask to the manometer to measure the pressure.
  2. Measure the temperature and pressure:
    • Record the initial temperature of the liquid in the boiling flask.
    • Record the initial pressure using the manometer or pressure gauge.
  3. Boil the liquid:
    • Heat the liquid in the boiling flask until it starts boiling.
    • Allow the gas to evolve and collect it in the gas collection apparatus.
  4. Measure the volume of gas:
    • Measure the volume of the evolved gas.

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    • Ensure the volume is measured under the same temperature and pressure conditions as the initial measurements.
  5. Calculate vapor density:
    • Vapor density (D) is defined as the ratio of the mass of a certain volume of the vapor to the mass of an equal volume of hydrogen at the same temperature and pressure.

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    • Calculate the vapor density using the formula: =Mass of gasVolume of gasD=Volume of gasMass of gas​
  6. Determine molar mass:
    • The vapor density is related to the molar mass (M) by the equation: Molar mass of hydrogenD=Molar mass of hydrogenM​
    • Rearrange the equation to solve for the molar mass of the volatile liquid: ×Molar mass of hydrogenM=D×Molar mass of hydrogen


  • Ensure that the measurements are taken under similar conditions of temperature and pressure.
  • The molar mass of hydrogen is approximately 2 g/mol.
  • The ideal gas law (PV = nRT) can be used to relate pressure, volume, and temperature.

Keep in mind that this method assumes ideal gas behavior, and corrections may be needed for non-ideal behavior or if the liquid undergoes decomposition. Safety precautions should be followed when working with volatile substances and heating equipment.

This experiment focused on determining the molar mass of a volatile liquid using a simple method based on the ideal gas law (PV = nRT). The approach involved measuring temperature, pressure, and volume of the gas. The obtained experimental result for the molar mass was 49.64 g/mol, slightly higher than the known value of 46.08 g/mol.

Accurate determination of the molar mass of a volatile liquid is crucial in categorizing newly synthesized chemical compounds. The experiment utilized the ideal gas law as the fundamental principle for calculating the amount of gas based on measurable pressure, volume, and temperature.

Various materials, such as an iron clamp, aluminum foil, boiling chips, thermometer, Erlenmeyer flask, syringe, and beaker, were employed. The molar mass of the volatile liquid was determined by measuring temperature, pressure, mass, and volume. The experimental procedure involved injecting the unknown solution into a flask covered with aluminum foil, immersing the flask in a boiling water bath, and subsequent measurements.

Calculation of the molar mass relied on measuring pressure, volume, temperature, and mass of the volatile liquid. The obtained molar mass from the experiment was 49.64 g/mol, slightly deviating from the known value of 46.08 g/mol. The discrepancy may be attributed to experimental errors or assumptions made during the procedure.

Table 1. Weighing Data For Volume Of The Flask



Mass of flask with foil cover (g)


Mass of flask filled with water and covered with foil (g)


Mass of water (g)


Density of water (g/mL)


Table 1 presents the weighing data used to determine the volume of the flask. The mass of water was derived by subtracting the mass of the flask with foil from the mass of the flask with foil filled with water. The flask's volume was then calculated by multiplying the mass of water by the density of water, as indicated in the subsequent calculation:

Volume of flask (mL)=Mass of waterDensity of waterVolume of flask (mL)=Density of waterMass of water​

As the unknown liquid evaporated during the experiment, it displaced air molecules in the flask, resulting in a change in the flask's mass. This mass difference represents the mass of the gas molecules.

Table 2 displays the data used to compute the gas mass for Trials 1 and 2. The gas mass was obtained by subtracting the mass of the flask and foil from the mass of the flask, foil, and gas sample.

Table 3 exhibits the data used in calculating the molar mass of the volatile liquid, along with the calculated molar mass for Trials 1 and 2. The molar mass of the gas from the volatile liquid was determined using the formulas:

PVMolarMass=PVMass of gas×RT​

The mean molar mass was computed, resulting in a value of 49.64 g/mol. The unknown test solution was identified as ethanol, with a known molar mass of 46.08 g/mol. The percentage error was calculated using the formula:

Percentage Error=Actual Molar MassExperimental Molar Mass−Actual Molar Mass​×100

The observed error was attributed to the experiment's high-altitude location, leading to lower pressure and consequently yielding a higher molar mass than expected.

Conclusion and Recommendations:

The experimental results indicated a molar mass of 49.64 g/mol for the volatile liquid, 7.729% higher than the actual molar mass of ethanol. To enhance accuracy, the author recommends conducting more trials in future experiments.

Updated: Feb 29, 2024
Cite this page

Determination of Molar Mass in Volatile Liquids through Ideal Gas Law: Experimental Insights and Error Analysis. (2024, Feb 29). Retrieved from

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