Solutions

Solutions

Question

The freezing point of benzene decreases by 0.45°C when 0.2 g of acetic acid is added to 20 g of benzene. If acetic acid associates to form a dimer in benzene, percentage association of acetic acid in benzene will be
(Kf for benzene = 5.12 K kg mol–1)

  • 64.6% 

  • 80.4%

  • 74.6%

  • 94.6%

Answer

D.

94.6%

In benzene
2CH3COOH ⇌ (CH3COOH)2
straight i space equals space 1 space plus space open parentheses 1 half minus 1 close parentheses
straight i space minus space 1 minus straight alpha over 2
Here space straight alpha space is space degree space of space association
increment straight T subscript straight f space equals space iK subscript straight f straight m
0.45 space equals space open parentheses 1 minus straight alpha over 2 close parentheses left parenthesis 5.12 right parenthesis fraction numerator open parentheses begin display style fraction numerator 0.2 over denominator 60 end fraction end style close parentheses over denominator begin display style 20 over 1000 end style end fraction
1 minus straight alpha over 2 space equals space 0.527
straight alpha space equals space 0.945
percent sign space degree space of space association space equals space 94.5 percent sign

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