This topic turns a balanced equation into numbers you can use. You learn to find a relative formula mass, convert a mass to an amount in moles, use the equation ratio, and decide which reactant runs out first. Questions on it appear across the Cambridge IGCSE Chemistry 0620 course wherever a question asks “what mass” or “how much”.
It is the base for later calculation work in concentration, gas and yield calculations, so weak habits here get carried forward.
What should I know before starting?
You need to write and balance a formula and equation, because every calculation starts from a correct ratio. If that still feels uncertain, revisit formulas and equations first.
You also need comfortable arithmetic: dividing by two-digit numbers, rounding to three significant figures, and converting g to kg.
One orienting example
How many moles are in 4.8 g of magnesium? Take the relative atomic mass of Mg as 24.
The amount in moles is mass ÷ relative mass, so 4.8 ÷ 24 = 0.20 mol.
Now the equation 2Mg + O₂ → 2MgO says 2 mol of Mg forms 2 mol of MgO, a 1 : 1 ratio. So 0.20 mol of Mg forms 0.20 mol of MgO. With Mr of MgO = 24 + 16 = 40, the mass is 0.20 × 40 = 8.0 g.
Every lesson in this module is one piece of that chain: mass, amount, ratio, then back to mass.
In what order should I study the lessons?
- Calculate a relative formula mass: every later step uses an Mr, so get this exact first, including brackets.
- Convert mass to amount using consistent units: the amount = mass ÷ Mr step in both directions.
- Use an equation ratio to relate reacting amounts: the coefficients move you from one substance to another.
- Identify a limiting quantity from a simple reaction model: decide which reactant controls the amount of product.
- Check an amount calculation by units: a quick habit that catches errors in all the other steps.
Which traps catch most students?
- Forgetting to multiply everything inside a bracket, such as the OH in Ca(OH)₂.
- Using grams in a formula that needs kilograms, or the other way round.
- Comparing masses or raw moles instead of moles divided by the coefficient.
- Giving an answer with no unit, or with the wrong one.
How should I use the practice set?
Work the mixed practice set after the lessons, writing units on every line and covering the answer until you finish. Sort each wrong answer by type, and log the pattern with the mistake log and retest queue. The mole and equation-ratio tutor and equation balance reasoning trainer are useful for checking your steps.
Students who can follow a method but struggle to start it alone often benefit from online one-to-one Chemistry tuition. See the wider IGCSE Chemistry learning guide for where this module sits.