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JEE Advanced levelSingle correct optionDifficulty 3 of 3cb212

$3.00 g$ of ethanoic acid (molar mass $60 g mo l^{- 1}$ ) is dissolved in $100 g$ of benzene ( $K_{f} = 5.12 K kg mo l^{- 1}$ ). Assuming that the acid is completely dimerised through hydrogen bonding and does not ionise, the depression in freezing point is

A $2.56 K$
B $5.12 K$
C $1.28 K$
D $0.64 K$

Answer: C

Complete dimerisation halves the moles of solute:

n = (3.00/60) /2 = 0.025

mol of dimer. Molality

= 0.025/0.100 = 0.25 mol k g^{- 1}

.

Δ T_{f} = 5.12 \times 0.25 = 1.28 K

.

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