Question

Molecules of benzoic acid $\left(C_6{H}_{5}COOH\right)$ dimerise in benzene. '$w$' g of the acid dissolved in $30$g of benzene shows a depression in freezing point equal to $2$K. If the percentage association of the acid to form a dimer in the solution is $80$, then $w$ is:

(Given that $K_f=5$K kg $mo{l}^{-1}$, Molar mass of benzoic acid $=122$ g $mo{l}^{-1}$)

• A. $1.8$g
• B. $2.4$g
• C. $1.0$g
• D. $1.5$g

Answer 1

The correct option is B $2.4$g

$\underset{Wg}{2\left(C_6{H}_{5}COOH\right)}\stackrel{C_6{H}_6}{\underset{\left(30g\right)}{⟶}}\underset{\text{diser}}{C_6{H}_{5}COOH{)}_2}$

${\Delta }_{f}T=i\times k_f\times m$

$2=0.6\times 5\times \frac{w\times 1000}{122\times 30}$

$(i=1-0.8+0.4=0.6)$

$w=2.44$ g.

Hence, the correct option is $B$