3.9\,{g} of benzoic acid dissolved in ...

CBSE Class 12 Chemistry 2015 Solved Paper

Question : 16 of 26

Marks: +1, -0

3.9\,\mathrm{g}

of benzoic acid dissolved in

49\,\mathrm{g}

of benzene shows a depression in freezing point of

1.62\,\mathrm{K}

. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated).

(Given: Molar mass of benzoic acid

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

K_f

for benzene

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

)

Solution:

\Delta T_f\;=i K_f m

\Delta T_f\;=i K_f \;\frac{m_f \times 1000}{M_b \times m_a}

1.62\;=i \times 4.9\,\mathrm{K}\,\mathrm{kg}\,\mathrm{mol}^{-1} \times

\;\frac{3.9\,\mathrm{g}}{122\,\mathrm{g}\,\mathrm{mol}^{-1}} \times \frac{1000}{49\,\mathrm{kg}}

i\;=0.506

Or by any other correct method As

i < 1

, therefore solute gets associated.

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