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CBSE Class 12 Chemistry 2015 Solved Paper

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Question : 16 of 26
Marks: +1, -0
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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