Question

Acetic acid undergoes dimerisation, when dissolved in benzene (as shown in figure)
Molecular mass of acetic acid is found to be $120$.

Which among the following relation is correct?

(where, $\alpha$ = degree of association,

$d$ = observed vapour density,

$D$ = Theoretical vapour density).

• A. $\alpha =2\left(\frac{D-d}{d}\right)$
• B. $\alpha =2\left(\frac{D-d}{D}\right)$
• C. $\alpha =2\left(\frac{d-D}{d}\right)$
• D. $\alpha =\frac{2d}{D-d}$

Answer 1

Answer: (C)

$\alpha =2\left(\frac{d-D}{d}\right)$

Acetic acid undergoes dimerisation, when dissolved in benzene.
Molecular mass of acetic acid is found $120$.

The correct relation is $\alpha =2\left(\frac{d-D}{d}\right)$

where $d=$ Observed vapour density
$D=$ Theoretical vapour density
$\alpha =$ degree of association
$n=$ number of molecules that undergo association.

For association of molecule,

$\alpha =\frac{d-D}{d(1-\frac{1}{n})}$

Substitute $n=2$,

$\alpha =\frac{d-D}{d(1-\frac{1}{2})}$

$\alpha =\frac{d-D}{d\left(\frac{1}{2}\right)}$

$\alpha =2\left(\frac{d-D}{d}\right)$

Hence, option $\left(C\right)$ is the correct answer.