Amounts and Redox Problem Solving

This final lesson brings together everything you have learned about moles, equations, titrations, gas volumes, and redox chemistry. A-Level exam questions frequently require you to combine multiple concepts in a single multi-step calculation. The key to success is a systematic approach: identify what you know, write the balanced equation, and work step by step through the moles.

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Multi-Step Mole Calculations

Many exam questions present a chain of reactions or a scenario where you must move from one piece of information to the final answer through several intermediate steps.

Worked Example 1: From Mass to Gas Volume via Titration

A student dissolves 1.20 g of an impure sample of sodium carbonate (Na₂CO₃) in water and makes up the solution to 250 cm³. A 25.0 cm³ aliquot requires 18.5 cm³ of 0.100 mol dm⁻³ HCl for complete neutralisation. Calculate the purity of the sodium carbonate.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Step 1: Moles of HCl = 0.100 × 0.0185 = 1.85 × 10⁻³ mol

Step 2: Moles of Na₂CO₃ in 25.0 cm³ = 1.85 × 10⁻³ / 2 = 9.25 × 10⁻⁴ mol (1:2 ratio)