What is the concentration of a solution containing 2g of NaOH in 100cm3 of solution? [Na = 23, O =16, H = 1]

Analyze the question: We need the molar concentration (moldm⁻³) of NaOH when 2 g is dissolved to make 100 cm³ of solution.

Concept: Concentration (mol/dm³) = number of moles ÷ volume in dm³ Number of moles = mass ÷ molar mass

Solution:

1. Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol

2. Moles of NaOH = 2 ÷ 40 = 0.05 mol

3. Convert the volume to dm³: 100 cm³ = 100 ÷ 1000 = 0.1 dm³

4. Concentration = 0.05 ÷ 0.1 = 0.50 mol/dm³

Check using the other route (g/dm³): 2 g in 100 cm³ → 20 g in 1000 cm³ = 20 g/dm³ Concentration = 20 ÷ 40 = 0.50 mol/dm³ ✓

Answer: Option B, 0.50 moldm⁻³.

Note that option C (0.05) is the trap: it is the number of moles, and it is what you get if you forget to convert 100 cm³ to 0.1 dm³.

Tip: For these questions, remember the sequence mass → moles → divide by volume in dm³. Always convert cm³ to dm³ by dividing by 1000. A quick shortcut is to scale the mass up to 1000 cm³ (multiply by 10 here), then divide by the molar mass.