Formulas
Cambridge A-Level Chemistry (9701) · Unit 2: Atoms, molecules and stoichiometry · 10 flashcards
Formulas is topic 2.3 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 2 — Atoms, molecules and stoichiometry , alongside Relative masses of atoms and molecules and The mole and the Avogadro constant. In one line: Aluminum has a charge of +3 (Al³⁺) and oxygen has a charge of -2 (O²⁻). To balance the charges, the formula is Al₂O₃.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 10 flashcards — 7 definitions and 3 calculations — covering the precise wording mark schemes reward. Use the 7 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 2.4 Reacting masses and volumes (of solutions and gases). Each card is labelled with its topic.
Write the formula for aluminum oxide
Aluminum has a charge of +3 (Al³⁺) and oxygen has a charge of -2 (O²⁻). To balance the charges, the formula is Al₂O₃.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Write the formulas of ionic compounds from the charges on the ions and from oxidation numbers (written as Roman numerals), including: (a) predicting an ion's charge from where its element is in the Periodic Table; (b) recalling the name and formula of each of these ions: NO₃⁻, CO₃²⁻, SO₄²⁻, OH⁻, NH₄⁺, Zn²⁺, Ag⁺, HCO₃⁻, PO₄³⁻.
- (a) Write and construct balanced equations, including ionic equations, which must leave out spectator ions. (b) Use suitable state symbols in equations.
- Define the terms empirical formula and molecular formula, and use them.
- Understand the terms water of crystallisation, hydrated and anhydrous, and use them correctly.
- Use data provided to calculate empirical formulas and molecular formulas.
Also in this deck: 2.4 Reacting masses and volumes (of solutions and gases)
- Carry out calculations based on the mole concept involving: (a) reacting masses worked out from formulas and equations, including percentage yield; (b) gas volumes, for example when hydrocarbons burn; (c) solution volumes and concentrations; (d) the limiting reagent and the reagent in excess. Quote answers to the number of significant figures the question supplies or requests, rounding so that figures are neither lost needlessly nor claimed beyond what the data justify (the mathematical requirements also apply). (e) Deduce stoichiometric relationships from calculations like those in (a) to (d).
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › Always check the subscripts in a chemical formula to ensure the correct number of metal ions are used in quantitative calculations.
- › Check the question conditions carefully; use 22.4 dm³ per mole for calculations involving gas at s.t.p.
- › Identify all gaseous products, including water vapour, to determine the true mass of the remaining solid residue.
- › Always calculate moles for both reactants to identify the limiting reagent before proceeding to gas volume calculations using the balanced equation.
Topic 2.3 · Formulas
Write the formula for aluminum oxide.
Aluminum has a charge of +3 (Al³⁺) and oxygen has a charge of -2 (O²⁻). To balance the charges, the formula is Al₂O₃.
Topic 2.3 · Formulas
Write the formula for copper(II) sulfate.
Copper(II) indicates a +2 charge (Cu²⁺). Sulfate is SO₄²⁻. Therefore, the formula is CuSO₄.
Topic 2.3 · Formulas
Define empirical formula.
The empirical formula is the simplest whole number ratio of atoms of each element in a compound.
Topic 2.3 · Formulas
Define molecular formula.
The molecular formula is the actual number of atoms of each element in a molecule.
Topic 2.3 · Formulas
Define anhydrous.
Anhydrous means a substance contains no water molecules. It is the opposite of hydrated.
Topic 2.3 · Formulas
Define hydrated.
Hydrated means a substance contains water molecules within its crystal structure.
Topic 2.3 · Formulas
Define water of crystallisation.
Water of crystallisation refers to water molecules that are chemically bonded within the crystal structure of a hydrated compound.
Topic 2.3 · Formulas
A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Calculate its empirical formula.
Assume 100g, so 40g C, 6.7g H, 53.3g O. Convert to moles: C = 40/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33. Divide by smallest (3.33): C = 1, H = 2, O = 1. Empirical formula is CH₂O.
Topic 2.4 · Reacting masses and volumes (of solutions and gases)
0.100 g of magnesium reacts with excess hydrochloric acid to produce 0.00329 mol of hydrogen gas. What is the percentage yield of hydrogen?
Mg + 2HCl → MgCl₂ + H₂. Moles of Mg = 0.100 / 24.3 = 0.00412 mol, so the theoretical yield of H₂ = 0.00412 mol. % yield = (actual yield / theoretical yield) × 100 = (0.00329 / 0.00412) × 100 = 79.9%.
Topic 2.4 · Reacting masses and volumes (of solutions and gases)
25.0 cm³ of 0.100 mol dm⁻³ NaOH is neutralised by 20.0 cm³ of HCl. What is the concentration of the HCl?
NaOH + HCl → NaCl + H₂O. Moles of NaOH = (25/1000) * 0.1 = 0.0025 mol. Moles of HCl = moles of NaOH = 0.0025 mol. Concentration of HCl = moles/volume = 0.0025/(20/1000) = 0.125 mol dm⁻³
More Chemistry flashcards
Browse every 9701 flashcard topic by syllabus area.
All Chemistry FlashcardsMore topics in Unit 2 — Atoms, molecules and stoichiometry
Formulas sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Formulas deck
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