Which of the following aqueous solutions has the lowest vapour pressure at a given temperature?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 0.1 molal sodium phosphate (Na3PO4)
Lowering of vapour pressure is a colligative property: it depends on the number of solute particles in the solution. All four solutions are 0.1 molal, so the solute that gives the most particles per formula unit (the largest van’t Hoff factor i) lowers the vapour pressure most.
\( \ce{Na3PO4} \) — \( \ce{Na3PO4 -> 3Na^+ + PO4^3-} \), so i = 4.
\( \ce{BaCl2} \) — \( \ce{BaCl2 -> Ba^2+ + 2Cl^-} \), so i = 3.
\( \ce{NaCl} \) — \( \ce{NaCl -> Na^+ + Cl^-} \), so i = 2.
Glucose — a non-electrolyte; it does not dissociate, so i = 1.
Sodium phosphate gives the most particles, so its solution has the lowest vapour pressure.
NCERT (2026-27 edition):
1.6 Colligative Properties and Determination of Molar Mass – “We have learnt in Section 1.4.3 that the vapour pressure of solution decreases when a non-volatile solute is added to a volatile solvent. … All these properties depend on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution.”
1.7 Abnormal Molar Masses – “We know that ionic compounds when dissolved in water dissociate into cations and anions. For example, if we dissolve one mole of KCl (74.5 g) in water, we expect one mole each of K+ and Cl– ions to be released in the solution. If this happens, there would be two moles of particles in the solution.”