In a freezing point depression lab, the molecular weight of a substance can be determined by measuring the change in the freezing point of a solvent after the substance has been dissolved in it. The freezing point depression is directly proportional to the molality of the solution, which is the number of moles of solute per kilogram of solvent. By using the freezing point depression constant for the solvent and the molality of the solution, the molecular weight of the solute can be calculated.
To perform the freezing point depression lab, a known mass of the solute is dissolved in a measured volume of solvent. The freezing point of the pure solvent is then measured and the freezing point of the solvent after the solute has been added is also measured. The difference between these two temperatures is the freezing point depression.
The freezing point depression constant for the solvent is a value that is specific to each solvent and is used to convert the freezing point depression into a molality value. The molality is then used to calculate the molecular weight of the solute using the following equation:
Molecular weight of solute = molality x freezing point depression constant x solvent molecular weight
The accuracy of the molecular weight determination depends on the accuracy of the measurements and the purity of the solvent and solute. It is important to carefully control the temperature during the measurements and to use a solvent that is pure and free of impurities.
In conclusion, the determination of molecular weight by freezing point depression is a useful technique for determining the molecular weight of a substance. It relies on the relationship between the molality of a solution and the freezing point depression of the solvent, and allows for the calculation of the molecular weight of the solute using a simple equation. However, the accuracy of the determination depends on the purity of the solvent and solute and the precision of the measurements.