Solutions and Colligative Properties 2 Question 32
32. Nitrobenzene is formed as the major product along with a minor product in the reaction of benzene with a hot mixture of nitric acid and sulphuric acid. The minor product consists of carbon : $42.86 %$, hydrogen : $2.40 %$, nitrogen : $16.67 %$ and oxygen : $38.07 %$,
(i) Calculate the empirical formula of the minor product.
(ii) When $5.5 \mathrm{~g}$ of the minor product is dissolved in $45 \mathrm{~g}$ of benzene, the boiling point of the solution is $1.84^{\circ} \mathrm{C}$ higher than that of pure benzene. Calculate the molar mass of the minor product then determine its molecular and structural formula. (Molal boiling point elevation constant of benzene is $2.53 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$ ).
(1999)
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Solution:
- (i) Empirical formula determination
| Elements | $\mathbf{C}$ | $\mathbf{H}$ | $\mathbf{N}$ | $\mathbf{O}$ |
|---|---|---|---|---|
| Weight % | 42.86 | 2.40 | 16.67 | 38.07 |
| Moles | 3.57 | 2.40 | 1.19 | 2.38 |
| Simplest ratio | 3 | 2 | 1 | 2 |
$\Rightarrow$ Empirical formula $=\mathrm{C}{3} \mathrm{H}{2} \mathrm{NO}_{2}$
(ii) $\Delta T_{b}=1.84=2.53 \times \frac{5.5}{M} \times \frac{1000}{45}$
$$ \Rightarrow \quad M=168 $$
$\because$ Empirical formula weight (84) is half of molar mass, molecular formula is $\mathrm{C}{6} \mathrm{H}{4} \mathrm{~N}{2} \mathrm{O}{4}$ a dinitrobenzene :
