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LearnCambridge06202026–2028Papers 2, 4 · ExtendedStoichiometry

Papers 2, 4 · Module

Stoichiometry

3 subtopics≈ 125 min in total

Start the module: Formulae and equations Open graphing calculator

Formulae and equations
A chemical formula shows which elements a substance contains and in what ratio. A balanced equation has the same number of each type of atom on both sides, because atoms are rearranged, never created or destroyed. State symbols (s), (l), (g), (aq) show the physical state. Ionic equations show only the ions that actually react.
≈ 35 min
Relative masses and the mole
Relative formula mass $M_r$ is the sum of the relative atomic masses in a formula. A mole is the amount of substance containing $6.02\times10^{23}$ particles (the Avogadro constant); one mole has a mass in grams equal to $M_r$ . So $n=\frac{m}{M_r}$ . The empirical formula is the simplest whole-number ratio of atoms, found by dividing each mass by $A_r$ and simplifying.
≈ 40 min
Mole calculations
Every mole calculation follows the same path: convert what you know into moles, use the mole ratio from the balanced equation, then convert back into what you want. Gases: $n=\frac{V}{24}$ ( $\mathrm{dm^3}$ at room temperature and pressure). Solutions: $n=c\times V$ ( $V$ in $\mathrm{dm^3}$ ). Percentage yield $=\frac{\text{actual}}{\text{theoretical}}\times100$ .
≈ 50 min