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Cambridge IGCSE Chemistry · 0620
Chapter 3: Stoichiometry — Part 6
Topic 3.3d · Stoichiometric calculations, yield and purity
Reacting masses and limiting reactants
- Mole ratio from equations
- Coefficients in a balanced equation give the mole ratio of reactants and products. Use n = m ÷ Mr to convert masses to moles.
- Limiting reactant
- The reactant that is completely used up first, limiting the amount of product formed. The other reactant(s) are in excess.
- Finding the limiting reactant
- Calculate moles of each reactant. Compare with the equation ratio — the reactant that gives the fewest moles of product is limiting. Product yield is calculated from the limiting reactant only.
Exam Traps
- Do not assume the reactant with smaller mass is limiting — compare moles against the equation ratio.
- Do not calculate product from the excess reactant — only the limiting reactant determines maximum product.
Yield and purity
- Percentage yield
- % yield = (actual yield ÷ theoretical yield) × 100. Theoretical yield is the maximum product calculated from the limiting reactant.
- Why yield is less than 100%
- Incomplete reactions, side reactions, and loss of product during transfer, filtration, or purification.
- Percentage composition by mass
- % of element = (total Ar of that element in formula ÷ Mr of compound) × 100.
- Percentage purity
- % purity = (mass of pure substance ÷ total mass of sample) × 100. Used when a sample contains impurities.
Exam Traps
- Do not confuse % yield with % purity — yield compares actual to theoretical product; purity compares pure substance to total sample mass.
- Theoretical yield must be calculated from stoichiometry (limiting reactant), not from the actual mass obtained.
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