IGCSE Chemistry · Quantitative Chemistry

Moles, Molar Mass & Reacting Masses

Once you can balance an equation, you can calculate exactly how much product a reaction makes — in grams. This lesson builds the mole toolkit step by step, then tests it across three exams.

mass, m (g)

moles, n (mol)

molar mass, M (g/mol)

Cover what you want to find: n = m ÷ M, or m = n × M

1 · The counting problem

Why chemists count in moles

Atoms are far too small and far too numerous to count one by one. A single gram of carbon contains about 50,000,000,000,000,000,000,000 atoms. Chemists solve this the way we solve counting eggs: with a bulk unit.

The mole is that unit. One mole of anything contains 6.02 × 10²³ particles — a number called the Avogadro constant. Just as "a dozen" always means 12, "a mole" always means 6.02 × 10²³, whether you're counting atoms, molecules, or ions.

2 · The bridge from atoms to grams

Molar mass

You can't weigh a mole directly on a balance in a lab — you weigh grams. Molar mass (M) is the bridge: it's the mass of one mole of a substance, in grams per mole (g/mol), and it's numerically equal to the relative formula mass you already calculate from atomic masses.

Example: molar mass of water, H₂O

M(H₂O) = (2 × 1) + 16 = 18 g/mol

One mole of water (6.02 × 10²³ molecules) has a mass of exactly 18 g.

3 · The one formula you'll use constantly

Moles, mass, and molar mass

These three quantities are linked by a single formula. Learn it once and you can rearrange it either way.

n = m ÷ M

Finding moles from mass

How many moles are in 12 g of carbon? (A_r of C = 12)

n = 12 ÷ 12 = 1 mol

Finding mass from moles

What is the mass of 3 mol of carbon? (A_r of C = 12)

m = 3 × 12 = 36 g

4 · The real payoff

Reacting masses: predicting how much product forms

Here's why balancing equations mattered so much: the coefficients in a balanced equation give you the mole ratio between reactants and products. Combine that ratio with the mole formula, and you can predict the mass of product formed from any mass of reactant — or work backwards from a product to find how much reactant was used.

The method, in three moves: (1) convert the given mass to moles, (2) use the balanced equation's mole ratio to find moles of the substance you want, (3) convert those moles back to mass.

What "excess" means: questions often say a substance reacts with an excess of something else — for example, "6 g of magnesium reacts with excess hydrochloric acid." This means there is more than enough of the excess substance, so the named substance (magnesium) is completely used up. That's why you always start your calculation from the mass you were given, not from the substance described as being in excess.

5 · Method, step by step

Working through a reacting-mass problem together

Question: What mass of magnesium oxide forms when 6 g of magnesium burns completely in oxygen?

2Mg + O₂ → 2MgO

6 · More practice, worked in full

Two more fully worked examples

CaCO₃ → CaO + CO₂

Question: What mass of calcium oxide forms when 25 g of calcium carbonate fully decomposes?

Zn + 2HCl → ZnCl₂ + H₂

Question: What mass of hydrogen gas forms when 13 g of zinc reacts completely with excess hydrochloric acid?

7 · Widen your range

Common reacting-mass calculations you'll meet

These worked answers cover the reaction types most often examined. Use them as a quick reference.

8 · Before you move on

Common mistakes to avoid

Using an unbalanced equation — the mole ratio is only correct once the equation is balanced.
Reading the mole ratio the wrong way round (mixing up which coefficient belongs to which substance).
Forgetting to convert back to mass at the end, and leaving the answer in moles.
Using the wrong relative atomic mass, or forgetting a multiplying subscript inside a formula like Ca(OH)₂.
Mixing up mass and molar mass — molar mass is a fixed property of a substance; mass depends on how much you actually have.

9 · Check yourself

Quick check before the exam

Type your numeric answers. A small margin of rounding error is allowed.

Ready? Three exams are waiting — fundamentals, reacting-mass calculations, and mixed applied problems — one question at a time with instant feedback.

How this works: pick one of the three exams below. Questions appear one at a time — answer, press Check answer for instant feedback, then move on. Calculation questions may have several boxes (e.g. moles of A, moles of B, mass of B) — each correct box earns a mark, so partial working still scores.