CIE 9701 Chemistry Mole Calculations: Avoid These Exam Mistakes
Stoichiometry is one of the most transferable skills in Cambridge Chemistry 9701. The arithmetic is rarely the hardest part. Students lose marks by using the wrong chemical ratio, skipping a unit conversion or starting before the equation is balanced.
Start from the balanced equation
A balanced chemical equation gives the mole ratio, not the mass ratio. For example, 2Mg + O2 → 2MgO says two moles of magnesium react with one mole of oxygen to form two moles of magnesium oxide.
If the question gives a mass, first convert it to moles. Only then use the coefficients. Converting masses directly using the 2:1:2 ratio is incorrect because the substances have different molar masses.
Worked example: magnesium oxide
Suppose 4.8 g of magnesium reacts with excess oxygen. Taking the relative atomic mass of Mg as 24, the amount of magnesium is 4.8 ÷ 24 = 0.20 mol.
The equation gives a 1:1 mole ratio between Mg and MgO, so 0.20 mol of MgO forms. With MgO relative formula mass 40, its mass is 0.20 × 40 = 8.0 g. The important method is mass → moles → ratio → required mass.
When neither reactant is in excess
Calculate how many moles of each reactant you have. Divide each amount by its coefficient in the balanced equation. The smaller result identifies the limiting reagent and therefore fixes the maximum amount of product.
Avoid assuming that the reactant with the lower mass is limiting. What matters is the amount in moles relative to the reaction ratio.
Get units right every time
For a solution, amount in moles = concentration in mol dm⁻³ × volume in dm³. A volume of 25.0 cm³ is 0.0250 dm³, not 25.0 dm³. For idealised gases at room temperature and pressure, the usual molar volume is 24.0 dm³ mol⁻¹, but read the stated conditions first.
Relative atomic and molecular masses are ratios; a calculated mass needs an actual mass unit. Keep track of whether the question asks for grams, moles, percentage yield or concentration before rounding.
Turn the next wrong MCQ into a revision target
When you miss an option, write the exact step where your method departed from the chemistry: balancing, mole conversion, limiting reagent or arithmetic. That gives you a short, fixable mistake rather than an unhelpful label like 'weak at calculations'.
The EIE stoichiometry notes explain each method and link directly to Cambridge Paper 1 questions with feedback, so you can check whether you can apply it under exam conditions.
Continue revising with EIE
Read the syllabus-mapped material for the exact topic, then test your understanding. Use the linked practice resources where available and keep a short record of what you need to revisit.