CBSE Class 12 · 2006 · Delhi · SolutionsShort Answer

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(a) Why is the vapour pressure of a solution of glucose in water lower than that of water?
(b) A 6.90 M solution of KOH in water contains 30% by mass of KOH. Calculate the density of the KOH solution. (Molar mass of KOH = 56 g mol

^{- 1}

)

Solution

(a) Glucose is non-volatile. Its molecules occupy part of the liquid surface, so fewer water molecules escape per unit time; by Raoult's law

p = x_{1} p^{\circ}

with

x_{1} < 1

, hence

p < p^{\circ}

.
(b) 1 L of solution contains

6.90 \times 56 = 386.4

g of KOH.
This is 30% of the solution's mass, so

mass of 1 L = \frac{386.4}{0.30} = 1288 g

Density:

d = \frac{1288}{1000}

d = 1.29 g mL^{- 1}

CBSE Class 12 Board · 2006 · Delhi · Subjective

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