(a) A 10% solution (by mass) of sucrose in water has a freezing point of 269.15 K. Calculate the freezing point of a 10% glucose solution in water, if the freezing point of pure water is 273.15 K. (Given: molar mass of sucrose = 342 g mol, molar mass of glucose = 180 g mol)
(b) Define the following terms:
(i) Molality (m)
(ii) Abnormal molar mass
(b) Define the following terms:
(i) Molality (m)
(ii) Abnormal molar mass
Solution
(a) Sucrose solution: 10 g per 100 g solution → 90 g water.
Its depression = K, so the effective constant:
Glucose solution: 10 g per 100 g solution → 90 g water.
Depression:
(*) K implied by the question's own sucrose data is used, as intended.
(b)(i) Molality: the number of moles of solute dissolved per kilogram of solvent.
(ii) Abnormal molar mass: the molar mass obtained from a colligative property when it differs from the true molar mass — it arises when the solute associates (observed mass too high) or dissociates (observed mass too low) in solution.
Its depression = K, so the effective constant:
Glucose solution: 10 g per 100 g solution → 90 g water.
Depression:
(*) K implied by the question's own sucrose data is used, as intended.
(b)(i) Molality: the number of moles of solute dissolved per kilogram of solvent.
(ii) Abnormal molar mass: the molar mass obtained from a colligative property when it differs from the true molar mass — it arises when the solute associates (observed mass too high) or dissociates (observed mass too low) in solution.
CBSE Class 12 Board · 2017 · Subjective