CBSE Class 12 Chemistry 2020 Outside Delhi Set 1 Solved Paper

Question : 28

Total: 37

SECTION -C

The freezing point of a solution containing 5g of benzoic acid (M=122gmol−1) in 35g of benzene is depressed by 2.94K. What is the percentage association of benzoic acid if it forms a dimer in solution?
(Kf‌ for benzene ‌=4.9K‌kg‌mol−1)

Solution:

Observed molar mass of benzoic acid:
‌MB=‌

Kf×WB

WA×∆Tf

‌WB=5‌gm
‌WA=0.035‌kg
‌MB=‌

4.9×5

0.035×2.94

=‌

24.5

0.1029

‌=238gmol−1
‌Kf=4.9Kkgmol−1
‌∆Tf=2.94
‌‌ Normal molar mass of ‌C6H5‌COOH
‌=122gmol−1
‌i=‌

‌ normal molar mass ‌

‌ observed molar mass ‌

=‌

122

238

=0.513
‌%‌ of association of acid ‌(α)
‌2C6H5‌COOH⇌(C6H5‌COOH)2
‌n=2α=‌

i−1

1∕n−1

=‌

0.513−1

−1

=‌

(−0.487)

(−0.5)

=0.974
Percentage association of acid =0.974×100= 97.4%

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