MCQ
Given below are two statements :
Statement (I) : Molal depression constant $K _{ f }$ is given by $\frac{ M _1 R T_f}{\Delta S_{\text {fus }}}$, where symbols have their usual meaning.
Statement (II) : $K _{ f }$ for benzene is less than the $K _{ f }$ for water.
In the light of the above statements, choose the most appropriate answer from the options given below :
  • A
    Statement I is incorrect but Statement II is correct
  • B
    Both Statement I and Statement II are incorrect
  • C
    Both Statement I and Statement II are correct
  • Statement I is correct but Statement II is incorrect

Answer

Correct option: D.
Statement I is correct but Statement II is incorrect
(D) Statement I is correct but Statement II is incorrect
Statement-I
Molar depression constant $k _f=\frac{ M _1 RT _{ f }^2}{\Delta H _{\text {fus }}}$
$k _f=\frac{ M _1 RT _{ f }}{\left[\frac{\Delta H _{\text {fus }}}{ T _{ f }}\right]} $
$k _f=\frac{ M _1 RT _f}{\Delta S_{\text {fus }}}$
Hence statement-I is correct
but $k _{ f }$ for benzene $=5.12 \frac{{ }^{\circ} C }{\text { molal }}$
$k _{ f }$ for water $=1.86 \frac{{ }^{\circ} C }{\text { molal }}$ Hence statement- II is incorrect

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