Consider the following statements about the elevation of boiling point.
(A) A liquid boils at the temperature at which its vapour pressure is equal to the atmospheric pressure.
(B) At 373.15 K, the vapour pressure of an aqueous solution of sucrose is more than 1.013 bar.
(C) For dilute solutions, the elevation of boiling point is directly proportional to the molal concentration of the solute.
(D) The molal elevation constant, Kb, is also called the cryoscopic constant.
(E) The unit of Kb is K kg mol−1.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (C) and (E) only
A non-volatile solute lowers the vapour pressure of the solvent, so the solution must be heated to a higher temperature before its vapour pressure reaches the atmospheric pressure. This rise is the elevation of boiling point, ΔTb = Kbm.
(A) A liquid boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. — True. Water, for example, boils at 373.15 K because its vapour pressure there is 1.013 bar.
(B) At 373.15 K, the vapour pressure of an aqueous solution of sucrose is more than 1.013 bar. — False. Sucrose is non-volatile, so the solution has a vapour pressure less than 1.013 bar at 373.15 K; that is why it does not boil at this temperature.
(C) For dilute solutions, the elevation of boiling point is directly proportional to the molal concentration of the solute. — True. ΔTb ∝ m, which gives ΔTb = Kbm.
(D) The molal elevation constant, Kb, is also called the cryoscopic constant. — False. Kb is the ebullioscopic constant; the cryoscopic constant is Kf, the molal depression constant.
(E) The unit of Kb is K kg mol−1. — True. Kb = ΔTb/m has the unit K divided by mol kg−1, that is K kg mol−1.
NCERT (2026-27 edition):
1.6.2 Elevation of Boiling Point – “The vapour pressure of a liquid increases with increase of temperature. It boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. For example, water boils at 373.15 K (100° C) because at this temperature the vapour pressure of water is 1.013 bar (1 atmosphere). … For example, the vapour pressure of an aqueous solution of sucrose is less than 1.013 bar at 373.15 K.”
“Experiments have shown that for dilute solutions the elevation of boiling point (ΔTb) is directly proportional to the molal concentration of the solute in a solution. … Here m (molality) is the number of moles of solute dissolved in 1 kg of solvent and the constant of proportionality, Kb is called Boiling Point Elevation Constant or Molal Elevation Constant (Ebullioscopic Constant). The unit of Kb is K kg mol-1.”