If 0.20 mol $dm^{-3}$ NaOH(aq) was evaporated to yield 5.0g of solid NaOH, calculate the volume of NaOH (aq) used. (Na = 23.0,O = 16:0; H = 1.00)

Analyze the question: A NaOH solution of concentration 0.20 mol/dm³ was evaporated and gave 5.0 g of solid NaOH. We need the volume of solution that contained this mass of NaOH.

Concept:

• Moles = mass ÷ molar mass
• Volume = moles ÷ concentration

Solution:

1. Molar mass of NaOH = 23.0 + 16.0 + 1.00 = 40.0 g/mol

2. Moles of NaOH = 5.0 ÷ 40.0 = 0.125 mol

3. Volume = n ÷ C = 0.125 ÷ 0.20 = 0.625 dm³

4. Convert to cm³: 0.625 × 1000 = 625 cm³

Answer: The volume is 625 cm³, which is option B.

Tip: For "find the volume" questions, follow the chain mass → moles → volume. Remember that 1 dm³ = 1000 cm³, so multiply by 1000 at the end. Always check that your units match the options before choosing.