(a) Urea forms an ideal solution in water. Determine the vapour pressure of an aqueous solution containing 10% by mass of urea at 40°C. (Vapour pressure of water at 40°C = 55.3 mm Hg)
(b) Why is the freezing point depression of 0.1 M sodium chloride solution nearly twice that of 0.1 M glucose solution?
(b) Why is the freezing point depression of 0.1 M sodium chloride solution nearly twice that of 0.1 M glucose solution?
Answer: (a) p ≈ 53.5 mm Hg (b) NaCl gives two particles per formula unit, glucose one — ΔT_f counts particles.
Solution
(a) 10% by mass: 10 g urea + 90 g water.
Mole fraction of water:
By Raoult's law:
(b) Freezing-point depression is colligative. NaCl dissociates into Na + Cl (two particles per formula unit) while glucose remains molecular, so 0.1 M NaCl has nearly twice the particles — and nearly twice the — of 0.1 M glucose.
Mole fraction of water:
By Raoult's law:
(b) Freezing-point depression is colligative. NaCl dissociates into Na + Cl (two particles per formula unit) while glucose remains molecular, so 0.1 M NaCl has nearly twice the particles — and nearly twice the — of 0.1 M glucose.
CBSE Class 12 Board · 2006 · All India; also All India 2005 Compartment · Subjective