Which of the following is not an expression of Dalton's law of partial pressures?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(P = P_1^\circ x_1\)
Dalton’s law of partial pressures says that the total pressure over the solution is the sum of the partial pressures of the components. For two volatile liquids, Raoult’s law gives each partial pressure, and putting these into Dalton’s law gives the total pressure in terms of x2.
Option (1) \(P = P_1 + P_2\) — An expression of Dalton’s law. It is the law itself: total pressure = sum of the partial pressures.
Option (2) \(P = P_1^\circ x_1\) — Not an expression of Dalton’s law. This is Raoult’s law for one component: it gives only the partial pressure of component 1, \( p_1 = p_1^\circ x_1 \), not the total pressure of the mixture.
Option (3) \(P = P_1^\circ + (P_2^\circ - P_1^\circ)x_2\) — An expression of Dalton’s law. It is Dalton’s law with both partial pressures written by Raoult’s law and x1 = 1 − x2.
Option (4) \(P_1 = P - P_2\) — An expression of Dalton’s law. It is P = P1 + P2 rearranged to give one partial pressure.
NCERT (2026-27 edition):
Vapour Pressure of Liquid-Liquid Solutions – …
