Benzoic acid (true molar mass 122 g/mol) dimerises in benzene and shows an observed molar mass of 242 g/mol. The percentage association of the acid is approximately:
A99.2%
B49.6%
C75.0%
D50.4%
Answer & Solution
Correct answer: A. 99.2%
1. $i = $ normal / observed molar mass = $122 / 242 = 0.504$.
2. For dimerisation, total moles per starting mole = $1 - x/2$, so $i = 1 - x/2$.
3. $x/2 = 1 - 0.504 = 0.496$, hence $x = 0.992$.
4. Percentage association = $99.2\%$. (Option A is x/2, not x.)
_Source: NCERT Class 12 Chemistry Ch 1 "Solutions", p.25_
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