Short Answer

The Dumas method is an experimental technique used to determine the molecular weight of a volatile liquid. In this method, the liquid is heated until it turns into vapour inside a flask, and the mass of the vapour along with the temperature, volume, and pressure is measured. Using the ideal gas equation, the molecular weight is calculated.

This method is simple and widely used because it does not require sophisticated equipment. It is particularly useful for organic liquids that easily evaporate without decomposing at high temperatures.

Detailed Explanation

Dumas Method

The Dumas method is a classical laboratory technique used to find the molecular weight of volatile liquids—substances that evaporate easily when heated. Developed by the French chemist Jean Baptiste Dumas, this method uses basic principles of gas behaviour to determine the molecular mass accurately. It is based on heating a known amount of a liquid in a sealed container so that it completely vaporizes. Once the vapour fills the container, its mass and physical conditions (temperature and pressure) are used to calculate molecular weight.

The method works because gases follow predictable laws described by the ideal gas equation. By applying these laws, chemists can convert measured quantities into moles and ultimately determine molecular weight.

1. Principle of the Dumas method

The entire method depends on the ideal gas equation:

Where:
P = pressure of the vapour
V = volume of the flask
n = number of moles of vapour
R = gas constant
T = temperature (in Kelvin)

If the mass of the vapour is known, the molecular weight (M) can be calculated using:

This equation shows that molecular weight is directly obtained from experimental values.

2. Steps involved in the Dumas method

Even though the method is simple, it must be performed carefully to ensure accurate results.

1. Preparing the apparatus

A dry, light-weight flask (usually a glass boiling tube or round-bottom flask) is used. It is weighed empty to find its mass.

1. Adding the volatile liquid

A small amount of the test liquid is placed inside the flask. The flask is then sealed with foil or a stopper containing a tiny pinhole. The hole allows air to escape as the liquid vaporizes.

1. Heating the flask

The flask is immersed in a hot water bath or oil bath. The liquid inside evaporates completely and forces all air out. When only vapour remains, its temperature becomes equal to the bath temperature.

1. Cooling and weighing

After vaporization, the flask is removed, cooled, dried, and weighed again.
The difference between the new mass and the empty mass gives the mass of vapour.

1. Measuring volume and pressure

• The volume of the flask is measured (usually by filling it with water).
• Atmospheric pressure is noted using a barometer.

Using the measured mass, volume, temperature, and pressure, the molecular weight is calculated.

3. Why the Dumas method works well

The method is successful because:

• The volatile liquid completely vaporizes, ensuring accurate measurement.
• Air is expelled through the pinhole, so the mass measured corresponds only to vapour.
• Temperature and pressure are easy to measure accurately.
• The vapour behaves nearly like an ideal gas at high temperature.

This allows the ideal gas law to be applied without major errors.

4. Applications of the Dumas method

The Dumas method is used for:

• Determining molecular weight of volatile organic liquids
• Identifying unknown liquids
• Checking purity of chemicals
• Teaching gas laws in laboratory classes

It is particularly useful in educational settings because it uses simple equipment and demonstrates gas behaviour clearly.

5. Limitations of the Dumas method

Although effective, the method has some limitations:

• It only works for liquids that vaporize easily.
• Substances that decompose when heated cannot be used.
• Vapour must behave as an ideal gas for accurate results.
• Very high-boiling liquids cannot be studied with this method.

Despite these limitations, it remains a reliable and widely used technique in basic chemistry labs.

6. Comparison with other methods

The Dumas method is often compared with the Victor Meyer method.
Differences include:

• The Dumas method directly measures the mass of vapour, while Victor Meyer measures displaced air.
• Dumas method involves heating inside a container; Meyer method avoids decomposition by indirect heating.

Both methods are used for volatile substances, but Dumas method gives slightly more direct and accurate mass measurements.

Conclusion

The Dumas method is a classical and reliable technique for determining the molecular weight of volatile liquids. By heating the liquid until it vaporizes and applying the ideal gas equation to measured values of mass, temperature, pressure, and volume, chemists can calculate molecular weight accurately. The method is simple, educational, and widely used for organic liquids that evaporate easily. Although it has some limitations, it remains an important tool in laboratories for understanding gas behaviour and identifying unknown substances.