What is the concentration (mol dm$^{-3}$) of a solution containing 28 g of potassium hydroxide per 100 cm$^3$ of solution? (K=39; H=1; O=16)

Analyze the question: We need the molar concentration (mol dm⁻³) of KOH when 28 g is dissolved to make 100 cm³ of solution.

Concept: Concentration (mol dm⁻³) = number of moles ÷ volume in dm³ Number of moles = mass ÷ molar mass

Solution:

1. Molar mass of KOH = 39 + 16 + 1 = 56 g/mol

2. Moles of KOH = 28 ÷ 56 = 0.5 mol

3. Volume in dm³ = 100 cm³ ÷ 1000 = 0.1 dm³

4. Concentration = 0.5 ÷ 0.1 = 5.0 mol dm⁻³

Answer: The concentration is 5.0 mol dm⁻³, which is option D.

Tip: Convert cm³ to dm³ by dividing by 1000. A quick shortcut is to find the mass per 1000 cm³ first: 28 g × 10 = 280 g per dm³, then divide by the molar mass: 280 ÷ 56 = 5.0 mol dm⁻³. Option E (2.8) is a trap for students who forget to divide by the molar mass or mishandle the volume conversion.