CBSE Class 12 Chemistry 2015 Solved Paper

Question : 16 of 26

Marks: +1, -0

3.9 {\g}

of benzoic acid dissolved in

49 {\g}

of benzene shows a depression in freezing point of

1.62 {\K}

. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated).
(Given: Molar mass of benzoic acid

=122 {\g} {\mol}^{-1}

{\K}_f

for benzene

=4.9 {\K} {\kg} {\mol}^{-1}

)

Solution:

∆ {\T}_f\;=i {\K}_f {\m}

∆ {\T}_f\;=i {\K}_f \;{m_f × 1000}/{ {\M}_b × m_a}

1.62\;=i × 4.9 {\K} {\kg} {\mol}^{-1} ×

\;{3.9 {\g}}/{122 {\g} {\mol}^{-1}}× {1000}/{49 {\kg}}

i\;=0.506

Or by any other correct method As

i < 1

, therefore solute gets associated.

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