1. **Stating the problem:** Calculate the mass of sodium hypochlorite (NaClO) needed to prepare solution B, which has a volume of 500 mL and a concentration of 0.050 mol dm⁻³. 2. **Formula used:** The mass of solute is calculated by the formula: $$\text{mass} = \text{moles} \times \text{molar mass}$$ where moles can be found by: $$\text{moles} = \text{concentration} \times \text{volume}$$ 3. **Important rules:** - Volume must be in dm³ for concentration in mol dm⁻³. - Molar mass of NaClO is calculated by summing atomic masses: Na (22.99) + Cl (35.45) + O (16.00) = 74.44 g/mol. 4. **Intermediate work:** Convert volume from mL to dm³: $$500\ \text{mL} = 0.500\ \text{dm}^3$$ Calculate moles: $$\text{moles} = 0.050\ \text{mol/dm}^3 \times 0.500\ \text{dm}^3 = 0.025\ \text{mol}$$ Calculate mass: $$\text{mass} = 0.025\ \text{mol} \times 74.44\ \text{g/mol} = 1.861\ \text{g}$$ 5. **Final answer:** The mass of sodium hypochlorite needed to prepare solution B is **1.86 g** (rounded to two decimal places).