CIE A Level Chemistry Atoms, Molecules and Stoichiometry Exam Questions

About CIE A Level Chemistry Atoms, Molecules and Stoichiometry

Atoms, molecules and stoichiometry is the quantitative backbone of the Cambridge International 9701 course. It defines relative atomic, isotopic, molecular and formula masses on the carbon-12 scale, introduces the mole and the Avogadro constant, and asks you to write balanced and ionic equations with state symbols. Most of the marks come from calculations: reacting masses, empirical and molecular formulae, percentage yield, gas volumes and concentrations in titrations. This is AS content, but every later physical chemistry topic, from energetics to equilibria and electrochemistry, depends on confident mole calculations. Examiners look for clear working, correct units and answers given to an appropriate number of significant figures.

What you need to know

• Relative masses Relative atomic mass, Ar, is the weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12. Ar can be calculated from isotopic abundances, for example 75% chlorine-35 and 25% chlorine-37 gives 35.5.
• The mole One mole contains 6.02 x 10^23 particles (the Avogadro constant, L). Use n = m/M for masses, n = cV for solutions (V in dm3) and n = V/24.0 for gases at room conditions.
• Empirical and molecular formulae Divide each mass or percentage by Ar, then divide by the smallest value to find the simplest whole-number ratio. The molecular formula is a whole-number multiple of the empirical formula, found using Mr.
• Equations Balance equations and include state symbols. Ionic equations show only the species that change, for example H+(aq) + OH-(aq) → H2O(l). Spectator ions are left out.
• Reacting quantities Use the mole ratio from the balanced equation to link reactants and products. Calculate percentage yield as actual yield divided by theoretical yield x 100, and identify the limiting reagent when amounts are given for two reactants.
• Hydrated salts and titrations Water of crystallisation can be found from mass loss on heating. In titration calculations, find moles of the known solution, use the ratio, then scale up for any dilution or aliquot taken from a volumetric flask.

Common mistakes in Atoms, Molecules and Stoichiometry

• Using volume in cm3 in n = cV; convert to dm3 by dividing by 1000 first.
• Forgetting to use the mole ratio from the equation and assuming every reaction is 1:1.
• Rounding intermediate values too early, which gives a final answer outside the accepted range; keep extra figures until the end.
• Including spectator ions in an ionic equation or leaving out state symbols when asked for them.
• Defining relative atomic mass without saying it is a weighted mean or without reference to 1/12 of the mass of a carbon-12 atom.

Try three Atoms, Molecules and Stoichiometry exam-style questions