Question

Acetic acid dissolved in benzene shows a molecular weight of:
A. $60$
B. $120$
C. $180$
D. $240$

Answer 1

We know that some substances have ability to undergo association when they are dissolved in some specified solvent. Acetic acid is one of them that tends to dimerize when dissolved in benzene. This association leads to its abnormal molar mass.

Complete step by step answer:
We know that acetic acid dimerizes on dissolving in benzene. The formula of acetic acid is ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}}$. So the H-bonding between acetic acid and benzene leads to dimerization. We can write this as an equation:
${\rm{2C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}} \to {\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}}} \right)_2}$
The structure for dimerization is shown below.

We know that the normal molar mass of acetic acid is $2\left( {12} \right) + 4\left( 1 \right) + 2\left( {16} \right) = 60$ .So when two molecules of acetic acid are associated, the molar mass will become $2 \times 60 = 120$.
Thus, the correct option is B.

Note:
We know about abnormal molar mass that affects the colligative properties. This abnormality in mass arises whenever intermolecular forces between solutes are different from the intermolecular forces between solute-solvent. If the new interactions are stronger, the solute will undergo association and its molecular mass will also get increased.