Explain why we use molality and not molarity in depression of freezing point and elevation in boiling point?

We use molality instead of molarity in calculations involving the depression of freezing point and elevation in boiling point because molality is a more accurate measure of the concentration of solute particles in a solution.

Molality is defined as the number of moles of solute dissolved in 1 kilogram of solvent, while molarity is defined as the number of moles of solute dissolved in 1 liter of solution. When calculating the freezing point depression or boiling point elevation, it is important to consider the number of solute particles present in the solution, as this affects the colligative properties of the solution.

Since molality takes into account the mass of the solvent, it provides a more accurate measure of the concentration of solute particles in the solution. This is particularly important in colligative properties calculations, where the number of solute particles is directly related to the extent of the freezing point depression or boiling point elevation.

Therefore, molality is preferred over molarity in these calculations to ensure accurate results and to account for the effects of solute particles on the colligative properties of the solution.

Molality is preferred over molarity for determining the depression of freezing point and elevation in boiling point because it is a measure of the number of solute particles present relative to the mass of the solvent, rather than the volume of the solution. This is important because the freezing point and boiling point of a solution are determined by the concentration of the solute particles, not the volume of the solution.

Molarity, on the other hand, is a measure of the number of solute particles present relative to the volume of the solution. This can be misleading when dealing with solutions that have different densities. For example, a solution with a high density will have a lower volume than a solution with a low density, even if they contain the same number of solute particles. This can lead to errors when using molarity to determine the freezing point and boiling point of a solution.

Molality, however, is independent of the density of the solution. This means that a solution with a high density will have the same molality as a solution with a low density, even if they contain different volumes of solvent. This makes molality a more accurate measure of the concentration of solute particles in a solution, and therefore a more reliable way to determine the freezing point and boiling point of a solution.