Question

1. Explain/Discuss what a colligative property is. Provide at least 2 examples of colligative properties.

2. Provide at least one real-life application of freezing point depression.

3. Define the concentration term molality (m). Under what conditions are Molarity and molality approximately the same?

4. Discuss/Explain why the temperature of a substance remains relatively constant during the freezing of liquid to solid.

Answer 1

1. Colligative properties of solutions are properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute. These characteristics are fully dependent on the number of particles dissolved in the solution. It does not matter what those particles are. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

2. Freezing Point Depression: It is the process in which addition of a solute to a solvent decreases the freezing point of the solvent.

Example: i)Addition of salt (solute) to water decreases the freezing point of water (solvent).

ii) sea water, (a mixture of salt [and other things] in water) is able to remain liquid at temperatures below 0 °C (32 °F), the freezing point of pure water because of this process.

3) Molality is defined as the no. of moles of solute per kilogram of solvent. Density of water at 25°C is around 1kg/L. So, when calculating molarity , which is the no.of moles of solute per litre of solvent, the conversion of volume to kgs is negligible. Thus the molality is approximately equal to molarity for dilute aqueous solutions at given temperature.