wt. of ${N}=\left(\frac{42}{100} \times 0.8\right)\, {gm}$

mole of $N=\frac{42 \times 0.8}{100 \times 14}=\frac{2.4}{100}\, {~mol}$

moles of ${NH}_{3}=\frac{2.4}{100}$

$2 {NH}_{3}+{H}_{2} {SO}_{4} \rightarrow\left({NH}_{4}\right)_{2} {SO}_{4}$

$\quad\downarrow\quad\quad\quad\downarrow$

$\frac{2.4}{100}$ $mole$ $\quad\frac{1.2}{100}$ $mole$

For ${H}_{2} {SO}_{4} \quad \frac{1.2}{100}=1 \times {V}(\ell)$

$\Rightarrow {V}_{{H}_{2} {SO}_{4}}=\frac{1.2}{100} \,\ell=12 \,{~m} \ell$