Skip to content

Salam Academy

Sign in Create account

LearnCambridge97012025–2027Paper 2 · AS Level Structured QuestionsAtoms, molecules and stoichiometry

Paper 2 · Module

Atoms, molecules and stoichiometry

2 subtopics≈ 95 min in total

Start the module: The mole and equations Open graphing calculator

The mole and equations
Relative masses are measured against one-twelfth of a carbon-12 atom. One mole contains $6.02\times10^{23}$ particles (the Avogadro constant, $L$ ), and has a mass in grams equal to $M_r$ : $n=\frac{m}{M}$ . Empirical formulae come from composition by mass (or combustion data); the molecular formula is a whole-number multiple matching $M_r$ . Equations balance atoms and charge; ionic equations omit spectator ions. Water of crystallisation is found from mass lost on heating. Atom economy $=\frac{M_r\text{ of desired product}}{\sum M_r\text{ of all products}}\times100$ .
≈ 45 min
Reacting quantities and titrations
Every calculation runs through moles: mass ( $n=\frac{m}{M}$ ), solution ( $n=cV$ , $V$ in dm³), gas ( $n=\frac{V}{24.0}$ dm³ at r.t.p.). Use the balanced equation's mole ratio to link substances. The limiting reagent is used up first and sets the yield. Titrations find unknown concentrations. In a back titration, an excess of reagent is added and the leftover is titrated; subtracting gives the amount that reacted. Percentage yield $=\frac{\text{actual}}{\text{theoretical}}\times100$ .
≈ 50 min