// CHEMISTRY L4 — STOICHIOMETRY
The recipe maths of chemistry.
A balanced equation is a recipe in ratios. Weigh out a reactant and the equation tells you — exactly — how much product you'll get. Turn the dial and follow every step of the calculation.
// FIG. 01 — THE REACTING-MASS CALCULATOR
{{ equation }}
{{ s.tag }}
{{ s.desc }}
{{ s.val }}
YOU WILL MAKE
{{ answer }} g
OF {{ findName }}
// THE MOLE — CHEMISTRY'S COUNTING UNIT
Atoms are far too small and too numerous to count one by one, so chemists count them in moles. One mole is 6.02 × 10²³ particles (Avogadro's number) — and, handily, one mole of any substance has a mass in grams equal to its relative formula mass (Mr). So 1 mol of water (Mr 18) weighs 18 g. The bridge between the lab (grams) and the equation (ratios) is one formula:
moles = mass (g) ÷ Mr
// THE FOUR-STEP METHOD
{{ m.n }}
{{ m.title }}
{{ m.text }}
// CONCENTRATION & SOLUTIONS
For reactions in solution we measure amount as concentration — moles of solute per cubic decimetre of solution (mol/dm³, where 1 dm³ = 1 litre = 1000 cm³): concentration = moles ÷ volume. This is the backbone of titration calculations, where you find an unknown concentration by reacting it with a solution you already know.
// LIMITING REACTANTS & YIELD
In a real reaction one reactant usually runs out first — the limiting reactant — and it alone decides how much product forms; the rest is left over in excess. And you rarely get every gram the maths predicts: the percentage yield compares what you actually made to the theoretical maximum, lost to spills, side-reactions and incomplete reactions.
// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).