2 g of benzoic acid (true molar mass 122 g/mol) in 25 g of benzene gives a freezing point depression of 1.62 K. With Kf = 4.9 K kg/mol, the experimentally observed molar mass of benzoic acid is approximately:
A242 g/mol
B484 g/mol
C61 g/mol
D121 g/mol
Answer & Solution
Correct answer: A. 242 g/mol
1. $M_2 = (K_f \times w_2 \times 1000)/(\Delta T_f \times w_1)$.
2. = $(4.9 \times 2 \times 1000)/(1.62 \times 25)$.
3. = $9800 / 40.5$.
4. $\approx 242$ g/mol. The value is about twice the true 122 g/mol, indicating dimerisation.
_Source: NCERT Class 12 Chemistry Ch 1 "Solutions", p.24_
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