Determination of Molar Mass – Class 12 | Chapter – 2 | Chemistry Short Notes Series PDF for NEET & JEE
Determination of Molar Mass: Colligative properties can be used to determine the molar mass of a solute in a solution. There are several colligative properties that can be used for this purpose, including vapour pressure lowering, boiling point elevation, and freezing point depression.
The basic principle behind the use of colligative properties for molar mass determination is that the extent of the property change is proportional to the number of solute particles in the solution, not their identity. Therefore, if the mass of solute added to a known mass of solvent is known, and the change in the colligative property is measured, the molar mass of the solute can be calculated.
Determination of Molar Mass
To determine the molar mass of a solute using freezing point depression:
- Measure the freezing point of the pure solvent (e.g. water).
- Add a known mass of the solute to a known mass of the solvent, and dissolve it completely.
- Measure the freezing point of the solution.
- Calculate the change in freezing point (ΔTf) using the equation: ΔTf = Tf(solvent) – Tf(solution).
- Calculate the molality (m) of the solution using the equation: m = moles of solute / mass of solvent (in kg).
- Calculate the molar mass of the solute using the equation: M = (Kf x w2) / (ΔTf x m), where Kf is the freezing point depression constant for the solvent, w2 is the mass of the solute added, and M is the molar mass of the solute.
It is important to note that colligative properties are only accurate for very dilute solutions, where the solute does not significantly interact with the solvent or other solute particles. In addition, the accuracy of the method can be affected by experimental errors such as impurities in the solvents, measurement errors, and temperature fluctuations.
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