SATHEE: Solutions Question 260

Solutions Question 260

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

Options:

A) 28.5%

B) 32.6%

C) 73.3%

D) 82%

Show Answer

Answer:

Correct Answer: C

Solution:

Observed mol. mass $= \frac{100 \times k_{f} \times w}{W \times Δ T_{f}}$

$= \frac{1000 \times 5.12 \times 20 \times 10^{- 3}}{1 \times 0.69} = 148.4$ Normal mol, mass of phenol=94 $i = \frac{N o r m a l m o l . m a s s}{O b s e r v e d m o l . m a s s} = \frac{94}{148.4} = 0.633$ Since, phenol is dimerized
$∴ n = 2$ Also, $i = 1 + [\frac{1}{n} - 1] α = 1 + [\frac{1}{2} - 1] α = 1 - \frac{α}{2}$
$\Rightarrow 0.633 = 1 - \frac{α}{2}$
$\Rightarrow \frac{α}{2} = 0.367$
$\Rightarrow α = 0.733 \times 100 = 73.3$

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Solutions Question 260

Question: Phenol associates in benzene to a certain extent to form a dimer. A solution containing 20×10−3 kg of phenol in 1.0 kg-of benzene has its freezing point depressed by 0.69 K. Calculate the fraction of phenol that has dimerised. ( Kf for benzene = 5.12 K kg mol−1 )

Options:

Answer:

Solution:

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