Question

Phenol associates in benzene to a certain extent to form dimer. A solution containing ${20\times 10}^{-3}$kg of phenol in $1.0$ kg of benzene has its freezing point decreased by $0.69K.$ The fraction of the phenol that has dimerised $(K_f$ of benzene is $5.12K{mol}^{-1}$) is:

• A. $18\%$
• B. $29\%$
• C. $46\%$
• D. $73\%$

Answer 1

Answer: (D)

$73\%$

$i=\frac{\Delta T_f\times W_A\times M}{K_f\times W_B}=\frac{0.69\times 1\times 94}{5.12\times 20}$

$\Rightarrow i=0.6334$

$\underset{1-\alpha }{2C_6{H}_{5}OH}\rightleftharpoons \underset{\frac{\alpha }{2}}{(C_6{H}_{5}OH{)}_2}$

$i=1-\alpha +\frac{\alpha }{2}=1-\frac{\alpha }{2}=0.6334$

$\therefore \alpha =0.73=73$%