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Molarity

Phenol associ...

Question

Phenol associates in benzene to certain extent to form a dimer.A solution containing $20 \times 10^{- 3} k g$ of phenol in 1.0 kg of benzene has its freezing point depressed by 0.69 K. Calculate the fraction of phenol that has dimerized .
( $K_{f}$ for benzene is $5.12 K k g {m o l}^{- 1}$ ).

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Solution

$2 C_{6} H_{5} O H ⇌ {(C_{6} H_{5} O H)}_{2}$
1 mole 0
$1 - α α / 2$
$T o t a l = 1 - α + \frac{α}{2} = 1 - \frac{α}{2}$
$i = \frac{1 - (α / 2)}{1} = 1 - \frac{α}{2}$
$Δ T_{f} = i K_{f} W_{2} \times 1000 M w_{2} \times W_{1}$
$0.69 = \frac{i \times 5.12 \times 20 \times 10^{3}}{94 \times 1.0 k g} k g$
$i = 1 - \frac{α}{2}, α = 0.733 =$ 73.3%

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