JEE Main 2017 · Chemistry · Solutions, Solid State and Surface Chemistry · medium

4 g of acetic acid is dissolved in 100 g of benzene and the freezing point is lowered by 1.28 K (Kf=5.12 K kg mol^-1). Assuming acetic acid…

Single correct option

2.4\ \mathrm{g}

of acetic acid is dissolved in

100\ \mathrm{g}

of benzene and the freezing point is lowered by

1.28\ \mathrm{K}

(

K_f=5.12\ \mathrm{K\,kg\,mol^{-1}}

). Assuming acetic acid associates only to dimers, the percentage association is

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Answer: (C) $75\%$

Moles

=2.4/60=0.04

; molality

=0.04/0.100=0.40

. Expected

\Delta T_f=5.12\times0.40=2.048

i=1.28/2.048=0.625

. For dimerisation

2\mathrm{A}\rightleftharpoons \mathrm{A_2}

i=1-\alpha/2

, so

\alpha=2(1-0.625)=0.75

, i.e.

75\%

. Taking

\alpha=1-i

gives

37.5\%

and

\alpha=i

gives

62.5\%

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