Question

The degree of dissociation of Ca(NO₃)₂ in a dilute aqueous solution containing 7.0 g of salt per 100 g of water at 100C is 70%. If vapour pressure of water at 100C is 760 mm Hg, the vapour pressure of the solution is:

• A. 735 mm Hg
• B. 730 mm Hg
• C. 760 mm Hg
• D. 746 mm Hg

Hint:

Use van’t Hoff factor when dissociation occurs.

Answer 1

The Correct Option is D

Moles of Ca(NO₃)₂: (7)/(164) ≈ 0.043 Moles of water: (100)/(18) ≈ 5.56 Ca(NO₃)₂ dissociates into 3 ions. van’t Hoff factor: i = 1 + (n-1)α = 1 + 2(0.7) = 2.4 Effective moles of solute: 0.043 × 2.4 = 0.103 Mole fraction of water: Xₑₓtwₐtₑᵣ = (5.56)/(5.56 + 0.103) ≈ 0.982 Vapour pressure of solution: P = Xₑₓtwₐtₑᵣ × P^ = 0.982 × 760 ≈ 746 mm Hg