CBSE Class 12 · 2011 · All India Compartment · SolutionsLong Answer

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a) List any four factors on which the colligative properties of a solution depend.
b) Calculate the boiling point of a one-molar aqueous solution of KBr (density 1.06 g mL

^{- 1}

). Given K

_{b}

for H

_{2}

O = 0.52 K kg mol

^{- 1}

; atomic masses: K = 39, Br = 80.

Solution

(a) Colligative properties depend on:
(i) the number of moles of solute particles in the solution,
(ii) the amount (mass) of the solvent,
(iii) whether the solute associates or dissociates (the van't Hoff factor

i

),
(iv) the nature of the solvent (through its constants

K_{b}

K_{f}

) and the temperature.
(b) 1 M KBr: 1 L of solution has mass

1000 \times 1.06 = 1060

g and contains

119

g of KBr.
Mass of water =

1060 - 119 = 941

g, so molality:

m = \frac{1}{0.941} = 1.063 mol kg^{- 1}

KBr

\to

^{+}

+ Br

^{-}

gives

i = 2

Δ T_{b} = i K_{b} m = 2 \times 0.52 \times 1.063 = 1.11 K

T_{b} = 100 + 1.11 = 101.11^{\circ} C

CBSE Class 12 Board · 2011 · All India Compartment · Subjective

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CBSE Class 12 · 2011 · All India Compartment · SolutionsLong Answer

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