Which of the following is not an expression of Dalton's law of partial pressures?

$P = P_1^\circ x_1$

The correct answer is Option (2) → $P = P_1^\circ x_1$

Dalton’s law of partial pressures states that the total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of individual gases.

So, (1) P = P₁ + P₂  (CORRECT)
This is the direct statement of Dalton’s law.

• (4) P₁ = P − P₂  CORRECT
  This is just a rearranged form of Dalton’s law.

• (2) P = P₁° x₁ INCORRECT
  This is Raoult’s law, which relates the partial pressure of a component in a liquid solution to its mole fraction. It is not related to Dalton’s law.

• (3) P = P₁° + (P₂° − P₁°) x₂ CORRECT
  This is derived from Raoult’s law but represents the total pressure of an ideal solution, which is still an expression for total pressure (though in solution context). It is not a standard Dalton form, but it still expresses total pressure in terms of components.
  So, Only option (2) does not represent Dalton’s law.