Question

True or False? Please, explain. (a) Addition of a solute at constant T and P to a pure solvent A always decreases µA. (b) The system’s temperature remains constant as pure water freezes at constant pressure. (c) The system’s temperature remains constant as an aqueous solution of sucrose freezes at constant pressure. (d) If a solute partly dimerizes in a solvent, the freezing-point depression is less than it would be if the solute did not dimerize. (e) The osmotic pressure is the pressure exerted on the semipermeable membrane by the solute molecules. (f) Addition of a tiny amount of a soluble impurity to a pure liquid always lowers the freezing point. (g) A liquid solution of two substances will always freeze entirely at one temperature. (h) For a binary (B + C) two-phase (α + β) system, n α B` α = n β B ` β , where ` α and ` β are the distances of a point on a tie line to the ends of a tie line. (i) In a two-phase, two-component liquid-vapor (`− v) system, xB = x v B + x ` B (j) The partial pressure PB of B vapor (assumed ideal) in equilibrium with a nonideal solution of B plus C must always increase when the B mole fraction in the solution is increased at constant temperature.

Answer 1

(a) Addition of a solute at constant T and P to a pure solvent A always decreases µA.

This is true. μA = μA^o + RT ln (xA) since, xA < 1 in a mixture, so μA < μA^o

(b) The system’s temperature remains constant as pure water freezes at constant pressure.

True

(c) The system’s temperature remains constant as an aqueous solution of sucrose freezes at constant pressure.

This is false, system’s temperature changes as an aqueous solution of sucrose freezes at constant pressure

(d) If a solute partly dimerizes in a solvent, the freezing-point depression is less than it would be if the solute did not dimerize.

This is true. ΔTf = KXB, where, XB = mole fraction of solute and K = depression constant

When solute dimerizes, XB decreases.

(e) The osmotic pressure is the pressure exerted on the semipermeable membrane by the solute molecules.

This is false. The osmotic pressure is the pressure exerted on the semipermeable membrane by the solvent molecules

(f) Addition of a tiny amount of a soluble impurity to a pure liquid always lowers the freezing point.

This is false.

(g) A liquid solution of two substances will always freeze entirely at one temperature.

This is false. A liquid solution of two substances need not always freeze entirely at one temperature

(h) For a binary (B + C) two-phase (α + β) system, n α B` α = n β B ` β , where ` α and ` β are the distances of a point on a tie line to the ends of a tie line.

True. Lever’s rule

(i) In a two-phase, two-component liquid-vapor (`− v) system, xB = x v B + x ` B

This is false. Here, the dependency is on the number of moles

(j) The partial pressure PB of B vapor (assumed ideal) in equilibrium with a nonideal solution of B plus C must always increase when the B mole fraction in the solution is increased at constant temperature.

This is true