CBSE Class 12 Chemistry 2015 Solved Paper

Question : 16

Total: 26

3.9g of benzoic acid dissolved in 49g of benzene shows a depression in freezing point of 1.62K. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated).
(Given: Molar mass of benzoic acid =122gmol−1, Kf for benzene =4.9K‌kg‌mol−1 )

Solution:

∆Tf‌=iKfm
∆Tf‌=iKf‌

mf×1000

Mb×ma

1.62‌=i×4.9K‌kg‌mol−1× ‌

3.9g

122gmol−1

1000

49‌kg

i‌=0.506
Or by any other correct method As i<1, therefore solute gets associated.

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