When Raoult’s law is regarded as a special case of Henry’s law, the H…

The correct answer is Option (4) → the vapour pressure of the pure component, p₁⁰

• Both laws say that the partial pressure of a volatile component is directly proportional to its mole fraction in the solution; only the constant is different:

      \( \displaystyle \text{Henry's law: } p = K_H\,x \qquad \text{Raoult's law: } p_1 = p_1^0\,x_1 \)

• Putting the two side by side, K_H takes the place of p₁⁰, the vapour pressure of the pure component.

Why the other options are incorrect

Option (1) p_total — Incorrect. The total pressure is the sum of the partial pressures of all the components; it is not the constant for one component.

Option (2) p₁ — Incorrect. p₁ is the quantity on the left of the law, which changes with x₁; the constant is p₁/x₁, which equals p₁⁰.

Option (3) 1/p₁⁰ — Incorrect. K_H has the unit of pressure (for example kbar), so it cannot be the reciprocal of a pressure.

NCERT (2026-27 edition):
1.4.2 Raoult’s Law as a special case of Henry’s Law – “…If we compare the equations for Raoult’s law and Henry’s law, it can be seen that the partial pressure of the volatile component or gas is directly proportional to its mole fraction in solution. Only the proportionality constant K_H differs from p₁⁰. Thus, Raoult’s law becomes a special case of Henry’s law in which K_H becomes equal to p₁⁰.”