How  much 83.4 % pure salt cake $N{a}_{2}S{O}_4$ could be produced  from 250 kg of 94.5 %  pure NaCl containing $Si{O}_2$as only impurity? 

94.5% of 250 Kg NaCl = 236.25 Kg
 $\text{2NaCl }\to {\text{ H}}_2{\text{SO}}_4\to {\text{ Na}}_2{\text{SO}}_4\text{ 2HCl}$
From equation236.25 Kg of NaCl produces 286.7Kg of salt cake.
 $\begin{array}{l}\frac{83.4}{100}=\frac{286.7}{x}\\ \therefore x=344Kg\end{array}$