Question

The electrolyte solutions show abnormal colligative properties. To account for this effect we define a quantity called the Van't Hoff factor given by :

$i=\frac{\text{Actual number of particles in solution after dissociation/association}}{\text{Number of formula units initially dissolved in solution}}$

$i=1$ (for non-electrolytes)

$i>1$ (for non-electrolytes, undergoing dissociation)

$i<1$ (for solutes, undergoing dissociation)

Answer the following questions:

A solution of benzoic acid is dissolved in benzene such that it undergoes molecular association and its molar mass approaches 244. The benzoic molecules will exist as :

• A. Dimer
• B. Monomer
• C. Tetramer
• D. Trimer

Answer 1

Answer: (A)

Dimer

Molecular formula of benzoic acid $=C_6{H}_{5}COOH$

Molecular mass of benzoic acid $=122$

Molecular mass of benzoic acid in the solution $=244$

Let,

Molecular formula of benzoic acid in the solution $=(C_6{H}_{5}COOH{)}_n$

Now,

$n=\frac{M_{(C_6{H}_{5}COOH{)}_n}}{M_{C_6{H}_{5}COOH}}=\frac{244}{122}=2$

Therefore, benzoic acid in the solution exist as dimer.