A chemist heats 29.25 g of sodium chloride (58.5 g/mol) with 49.0 g of concentrated sulphuric acid…

Step 1. Moles. NaCl: $\dfrac{29.25}{58.5} = 0.500\ \text{mol}$. H2SO4: $\dfrac{49.0}{98.0} = 0.500\ \text{mol}$.

Step 2. First stage. $\text{NaCl}\ +\ \text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{NaHSO}_{4}\ +\ \text{HCl}$ uses them 1 : 1, so $0.500\ \text{mol}$ of HCl forms. The volume is $0.500 \times 22.4\ \text{L} = 11.2\ \text{L}$.

Step 3. The hydrogen sulphate. $0.500\ \text{mol}$ of NaHSO4 has a mass of $0.500 \times 120.0\ \text{g/mol} = 60.0\ \text{g}$.

Step 4. Second stage. $\text{NaHSO}_{4}\ +\ \text{NaCl}\ \rightarrow \ \text{Na}_{2}\text{SO}_{4}\ +\ \text{HCl}$ uses the second $0.500\ \text{mol}$ of salt, and gives $0.500\ \text{mol}$ of Na2SO4. The mass is $0.500 \times 142.0\ \text{g/mol} = 71.0\ \text{g}$.