Comprehension:
Solutions
The solution which obeys Raoult's law over the entire range of concentration are known as ideal solutions and which does not obey are known as non-ideal solution. The vapour pressure of a non ideal solution is either higher or lower than that predicted by Raoult's law equation. The colligative properties, which depend on the number of solute particles irrespective of their nature, are relative lowering of vapour pressure, depression of freezing point, elevation of boiling point and osmotic pressure. All these properties can be utilised for the determination of molar mass of the solute.
The elevation of boiling point of a solvent depends upon
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → the number of solute molecules.
Note: The given answer is as per the official NTA Answer Key. However, there is more than one correct option for this question, as explained below.
Option (3) the number of solute molecules is correct because elevation of boiling point is a colligative property: for a given solvent it depends on the number of solute particles, not on their nature.
Option (2) the nature of the solvent is also correct because the constant Kb in ΔTb = Kbm is a property of the solvent: the same molality of solute raises the boiling point of water by 0.52 K per mol kg−1, but that of benzene by 2.53 K.
m counts the moles (number) of solute particles per kilogram of solvent; the identity of the solute does not appear.
Kb, the molal elevation constant (ebullioscopic constant), depends only on the solvent:
\( \displaystyle K_b=\frac{R\times M_1\times T_b^2}{1000\times\Delta_{vap}H} \)
So, for a given solvent, the elevation of boiling point depends on the number of solute molecules.
Why the other options are incorrect
Option (1) the nature of the solute — Incorrect. 1 mol of glucose and 1 mol of urea in 1 kg of water raise the boiling point by the same amount, so the nature of the solute does not matter (as long as it does not dissociate or associate).
Option (4) the number of solvent molecules — Incorrect. The solvent enters only as a fixed quantity (1 kg) in the definition of molality; the elevation is decided by how many solute particles that amount of solvent contains.
NCERT (2026-27 edition):
Elevation of Boiling Point – …
