Short Answer

Molecular weight can be determined experimentally using different laboratory methods depending on the type of substance. For volatile substances, gas laws are used, while for non-volatile solutes, methods like freezing point depression, boiling point elevation, and osmotic pressure are commonly used. These techniques measure changes in physical properties to calculate molecular weight.

These experimental methods help chemists confirm the molecular weight of unknown substances and study the behaviour of compounds in solutions. They are especially useful in research, industries, and quality control where accurate molecular weight information is needed.

Detailed Explanation

Molecular Weight Determination

Experimental determination of molecular weight is an important part of chemistry because it helps identify substances, analyse reactions, and study physical and chemical properties. While molecular weight can be calculated from atomic masses, experimental methods allow us to measure it when the formula of a substance is unknown or needs verification. Different techniques are used depending on whether the substance is a gas, a volatile liquid, or a non-volatile solute.

These methods rely on the fact that molecular weight affects measurable physical properties such as boiling point, freezing point, vapour pressure, and osmotic pressure. By observing changes in these properties, the molecular weight can be calculated accurately.

1. Determining molecular weight using gas methods

For substances that readily vaporize, gases follow predictable behaviour. Gas laws allow chemists to calculate molecular weight based on measured values of pressure, volume, temperature, and mass.

1. Dumas method

In this classical method, a volatile liquid is heated until it becomes a gas inside a sealed flask. The mass of the vapour, the temperature, and the pressure are measured. Using the ideal gas equation (PV = nRT), the number of moles is calculated.
Then molecular weight = mass of vapour ÷ moles of vapour.

This method works well for organic liquids with low boiling points.

1. Victor Meyer method

In this technique, a volatile liquid is evaporated in a special apparatus. The displaced air volume is measured. From this volume, the number of moles of vapour is calculated, and molecular weight is determined.
This method is simple and avoids decomposition of heat-sensitive liquids.

2. Determining molecular weight using colligative properties

Colligative properties depend on the number of solute particles and not on their nature. These properties change when a solute is added to a solvent, and the change is related to molecular weight.

The main colligative property methods are:

1. Freezing point depression method

When a solute is dissolved in a solvent, the freezing point decreases.
The difference in freezing point is measured using a cryoscope.
The formula ΔTf = Kf × m is used, where m is molality.
From this, molecular weight is calculated because molality depends on moles of solute.

This method is often used for molar mass determination of polymers, organic compounds, and biological molecules.

1. Boiling point elevation method

Boiling point rises when a non-volatile solute is added.
ΔTb = Kb × m is used to calculate molality, which leads to molecular weight.
This method is useful when freezing point depression cannot be applied.

1. Osmotic pressure method

When a solution is separated from pure solvent by a semipermeable membrane, solvent moves across until pressure builds up—this is osmotic pressure.
Formula: π = MRT
From osmotic pressure (π), molecular weight can be calculated because M depends on the number of moles of solute.

This method is extremely accurate and widely used for large molecules such as proteins and polymers.

3. Vapour density method

For gases, vapour density (VD) can be measured experimentally.
The relation is:
Molecular weight = 2 × vapour density (for gases compared to hydrogen).

This simple method was historically important and is still used in basic laboratories.

4. Mass spectrometry

This modern technique directly measures the mass of molecules.
A sample is ionized, and the resulting ions are separated based on their mass-to-charge ratio.
The instrument gives a spectrum that shows the molecular mass very precisely.

Mass spectrometry is widely used for:

• Organic compounds
• Pharmaceuticals
• Environmental samples
• Biological molecules

It is one of the most accurate methods available.

5. Light scattering and osmometry for polymers

Large molecules like plastics and proteins cannot be measured easily using simple colligative property methods. For these substances:

• Light scattering measures how solute particles scatter light, which is related to molar mass.
• Osmometry measures osmotic pressure at very low concentrations for high-molecular-weight substances.

These advanced methods allow industries to study polymer strength, flexibility, and behaviour.

Why experimental determination is important

Experimental molecular weight determination is useful because:

• It confirms unknown substances.
• It detects impurities or unexpected changes in composition.
• It is needed for industries like pharmaceuticals, fertilizers, and polymers.
• It helps in research to understand new compounds.

Even when molecular formulas exist, experimental verification prevents errors and ensures safety and accuracy.

Conclusion

Molecular weight can be determined experimentally through several methods depending on the type of substance. Gas methods like the Dumas and Victor Meyer methods are used for volatile compounds. Colligative properties such as freezing point depression, boiling point elevation, and osmotic pressure help determine molecular weight of non-volatile solutes. Advanced methods like mass spectrometry and light scattering provide highly accurate results, especially for large molecules. These techniques allow chemists and industries to identify materials, analyse reactions, and ensure product quality.