According to Rault’s law.

P₁ = x₁ P₁⁰

where,

P₁ = Partial pressure of solvent

P₁⁰ = vapour pressure of pure solvent

x₁ = mole fraction of solvent.

We can also write the above equation like that —

P₁⁰ – P₁ = P₁⁰ – x₁P₁⁰

⇒ ΔP₁ = P₁⁰ (1 – x₁) [ΔP₁ = P₁⁰ – P₁]

ΔP₁ = reduction in the vapour pressure of solvent (ΔP₁).

⇒ ΔP₁ = P₁⁰x₂ [x₂ = (1 – x₁)]

x₂ = mole fraction of solute.

Hence, ΔP₁ = P₁⁰x₂