Question

Pick the correct statement:

• A. Relative lowering of vapour pressure is independent of T.
• B. Osmotic pressure always depends on the nature of solute.
• C. Elevation of boiling point is independent of nature of the solvent.
• D. Lowering of freezing point is proportional to the molar concentration of solute.

Answer 1

The Correct Option is A

The correct statement is: Relative lowering of vapor pressure is independent of T.

Here's why:

• Relative Lowering of Vapor Pressure: According to Raoult's Law, the relative lowering of vapor pressure is equal to the mole fraction of the solute: $\frac{P_0 - P_s}{P_0} = x_{solute}$ where:
  • P₀ is the vapor pressure of the pure solvent
  • P_s is the vapor pressure of the solution
  • x_solute is the mole fraction of the solute
• Osmotic Pressure: Osmotic pressure (π) is given by: $\pi = iMRT$ While molarity (M) and temperature (T) are involved, the van't Hoff factor (i) *can* be affected by the nature of the solute (e.g., whether it's an electrolyte or a nonelectrolyte), but it isn't *always* dependent on the nature of solute. Strong electrolytes dissociate completely into ions, but weak electrolytes only partially dissociate, and non-electrolytes do not dissociate at all. Furthermore, even for electrolytes, *ion pairing* becomes significant at higher concentrations.
• Elevation of Boiling Point: The elevation of the boiling point (ΔT_b) is given by: $\Delta T_b = K_b m$ where:
  • K_b is the ebullioscopic constant, which *depends on the solvent*
  • m is the molality of the solution
• Lowering of Freezing Point: The depression of the freezing point (ΔT_f) is given by: $\Delta T_f = K_f m$ where:
  • K_f is the cryoscopic constant, which depends on the *solvent*
  • m is the molality of the solution. While, at lower concentrations, molality is approximately equal to molarity, the phrase "molar concentration" is less accurate than "molality". This is a more subtle point, and the more relevant point is that Kf depends on the solvent.

Correct Answer: Relative lowering of vapor pressure is independent of T.