What is molar mass? From g/mol to balancing equations, reacting masses, limiting reactants and % yield

Molar mass = the mass of one mole of a substance, in g/mol.

It is the same number as Ar (an element) or Mr (a compound). Water, H₂O, has Mr = 18, so its molar mass is 18 g/mol: one mole of water weighs 18 g.

The formula for the whole page: amount (mol) = mass (g) ÷ molar mass (g/mol).

Four steps: molar mass → balance the equation → reacting masses → limiting reactant and % yield.

Ar (relative atomic mass) = an element's atomic mass compared with 1/12 of a carbon-12 atom, averaged over its isotopes. Read it off the Periodic Table. No unit.

Mr = the sum of the Ar of every atom in the formula. No unit (for ionic compounds it is called relative formula mass, still written Mr).

Molar mass = the same number as Mr, with the unit g/mol.

Rule of thumb: if the question asks for Mr, no unit; if it asks for molar mass, write g/mol.

Split the formula into elements, count the atoms, multiply by Ar and add up. Pick a substance below: each one has its own favourite trap.

Both sides must have the same number of each kind of atom: atoms are not created or destroyed, only rearranged.

The one rule never to break: change only the numbers in front (coefficients), never the small numbers (subscripts).

Press + / − until every row turns green.

Changing a subscript: turning H₂O into H₂O₂ to fix the oxygens is wrong; it is now a different substance.

Miscounting brackets: Ca(OH)₂ has 2 O and 2 H, not one of each.

Not grouping ions: if SO₄ appears unchanged on both sides, count it as one block instead of S and O separately.

Starting with O: elements that appear in several substances (usually O and H) are easiest left until last.

Forgetting state symbols: IGCSE often wants (s) (l) (g) (aq). Add them once it balances.

Always three steps: mass → moles (divide by molar mass) · moles → moles (use the numbers in front from the balanced equation) · moles → mass (multiply by molar mass).

Example: 2Mg + O₂ → 2MgO. How much MgO from 48 g of Mg? The usual wrong answer is "2 : 2, so 48 g". Right: 48 ÷ 24 = 2 mol Mg → 2 mol MgO → 2 × 40 = 80 g.

The ratio in an equation is a ratio of moles, not grams. This is where most marks go. Step through it below.

On the Core tier: the syllabus has you work reacting masses in simple proportions without moles. All the mole calculations on this page are Extended.

The limiting reactant is the one that runs out first. The reaction stops when it is gone, so it decides how much product you get. Like sandwiches: 10 slices of bread and 3 slices of cheese make only 3 sandwiches; the cheese is limiting.

How to find it: work out the moles of each reactant, divide each by its own coefficient in the equation; the smaller result is the limiting one.

Trap: the reactant with the smaller mass is not necessarily the one that runs out. Try H₂ and O₂ below.

% yield = actual mass ÷ theoretical mass × 100. Don't flip it. Over 100% means the product is still wet or impure, not that you did extra well.

Is the equation balanced?

Did you use the whole molecule for paired gases (O₂ H₂ N₂ Cl₂)?

Did you apply the ratio to moles, not grams?

Did you keep the decimals and round only at the end (usually to 3 significant figures)?

Units: g, mol, g/mol, and kg or tonnes converted to match before you start.

Reading a fertiliser bag: urea sold in Thailand is labelled 46-0-0, meaning about 46% nitrogen. Check it: CO(NH₂)₂ has Mr = 60, nitrogen 2 × 14 = 28 → 46.7%. Ammonium nitrate, NH₄NO₃ (Mr 80), is only 35% nitrogen, so kilo for kilo urea gives more.

The carbon in a tank of fuel: treat petrol as pure octane, C₈H₁₈. 114 g burns to 8 × 44 = 352 g of CO₂, about 3.09 times the fuel's mass. A litre of petrol weighs about 0.74 kg, so it releases roughly 2.28 kg of CO₂ (our estimate). The gas outweighs the fuel because oxygen from the air is added.

Cooking and baking: scaling a recipe from one tray to three, or cutting the whole recipe because only 2 eggs are left, is the same ratio and limiting-reactant thinking.

Real jobs in factories, medicines and food: anyone who makes a substance has to calculate how much raw material to use, and % yield shows how much was lost. Low yield = wasted money and more waste. That's why this skill runs through pharmacy, chemical engineering and food science.

A-Level, IB and university: concentrations, titrations, equilibrium and electrochemistry all use this mole thinking. Get this step solid and later topics get much easier.

Still unsure what a mole is? Start with the mole article. The Stoichiometry topic in the course has stations for balancing on the scales, the crucible and titration to try for real, with questions whose working is checked step by step against the mark scheme. IGCSE Chemistry course 4,900 baht, paid once. The platform is expected to open around December 2026.

Content follows the Cambridge IGCSE Chemistry 0620 syllabus (exams 2026–2028), topics 3.1–3.3. Ar from the syllabus Periodic Table (Cu = 64, Cl = 35.5), checked 25 October 2026. 46-0-0 is the standard label on urea fertiliser. The CO₂ per litre figure is our estimate assuming pure octane, not an official value.