Calculations
Cambridge IGCSE Chemistry (0620) · Unit 3: Stoichiometry · 16 flashcards
Calculations is topic 3.3 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 3 — Stoichiometry , alongside Formulae and Equations. In one line: Concentration can be measured in grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Mol/dm³ is also known as molarity (M).
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 confirm this is a high-yield topic: Cambridge has set undefined questions worth 438 marks here (about 6.9% of all Chemistry marks across those years).
The deck below contains 16 flashcards — 3 definitions and 3 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.
What units are used to measure concentration
Concentration can be measured in grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Mol/dm³ is also known as molarity (M).
What the Cambridge 0620 syllabus says
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- State State that concentration can be measured in g/dm³ or mol/dm³
- State State that the mole, mol, is the unit of amount of substance and that one mole contains 6.02 x 10²³ particles, e.g. atoms, ions, molecules; this number is the Avogadro constant Supplement
- Use Use the relationship amount of substance (mol) = mass (g) / molar mass (g/mol) to calculate: (a) amount of substance (b) mass (c) molar mass (d) relative atomic mass or relative molecular/formula mass (e) number of particles, using the value of the Avogadro constant Supplement
- Use Use the molar gas volume, taken as 24 dm³ at room temperature and pressure, r.t.p., in calculations involving gases Supplement
- Calculate Calculate stoichiometric reacting masses, limiting reactants, volumes of gases at r.t.p., volumes of solutions and concentrations of solutions expressed in g/dm³ and mol/dm³, including conversion between cm³ and dm³ Supplement
- Use Use experimental data from a titration to calculate the moles of solute, or the concentration or volume of a solution Supplement
- Calculate Calculate empirical formulae and molecular formulae, given appropriate data Supplement
- Calculate Calculate percentage yield, percentage composition by mass and percentage purity, given appropriate data Supplement
What units are used to measure concentration?
Concentration can be measured in grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Mol/dm³ is also known as molarity (M).
What is a mole and what is the Avogadro constant?
A mole (mol) is the unit of amount of substance. One mole contains 6.02 x 10²³ particles (atoms, ions, molecules); this number is the Avogadro constant.
What is the formula relating amount of substance (mol), mass (g), and molar mass (g/mol)?
Amount of substance (mol) = Mass (g) / Molar mass (g/mol). This formula can be rearranged to solve for mass or molar mass.
How do you calculate the number of particles in a given amount of substance?
Number of particles = Amount of substance (mol) x Avogadro constant (6.02 x 10²³).
What is the molar gas volume at room temperature and pressure (r.t.p.)?
At room temperature and pressure (r.t.p.), the molar gas volume is taken as 24 dm³. This means one mole of any gas occupies 24 dm³ at r.t.p.
How do you calculate the mass of reactant needed to react completely with a given mass of another reactant?
1. Write the balanced equation. 2. Convert masses to moles. 3. Use the stoichiometry of the balanced equation to find the moles of the other reactant. 4. Convert moles back to mass.
What is a limiting reactant and how do you identify it?
The limiting reactant is the reactant that is completely used up in a reaction. To identify it, calculate the moles of each reactant and compare the mole ratio to the balanced equation. The reactant with the smallest mole ratio (compared to the balanced equation's requirement) is the limiting reactant.
How do you calculate the concentration of a solution in g/dm³ from mol/dm³ and vice-versa?
To convert from mol/dm³ to g/dm³, multiply the concentration in mol/dm³ by the molar mass of the solute. To convert from g/dm³ to mol/dm³, divide the concentration in g/dm³ by the molar mass of the solute.
Describe the steps in calculating the empirical formula from percentage composition.
1. Assume 100g sample (percentages become masses in grams). 2. Convert grams to moles by dividing by the atomic mass. 3. Divide each mole value by the smallest mole value obtained. 4. If necessary, multiply to get whole numbers. These whole numbers are the subscripts in the empirical formula.
How do you calculate percentage yield?
Percentage yield = (Actual yield / Theoretical yield) x 100%. The actual yield is the amount of product obtained experimentally. The theoretical yield is the amount of product calculated from the balanced equation assuming complete reaction.
How do you calculate percentage purity?
Percentage purity = (Mass of pure substance / Mass of impure substance) x 100%. This is used to determine the percentage of the desired compound in an impure sample.
Calculate the number of moles in 11.0 g of carbon dioxide (CO₂). (Mr of CO₂ = 44)
Moles = mass / molar mass
Moles = 11.0 / 44
Moles = 0.25 mol
Always check: mass must be in grams and molar mass in g/mol.
What volume does 0.50 mol of any gas occupy at room temperature and pressure (r.t.p.)?
At r.t.p., one mole of any gas occupies 24 dm³ (24,000 cm³).
Volume = moles × molar gas volume
Volume = 0.50 × 24
Volume = 12 dm³
This applies to all gases at r.t.p. regardless of their identity.
What is percentage yield and why is it usually less than 100%?
Percentage yield = (actual yield / theoretical yield) × 100
It is usually less than 100% because:
• The reaction may be reversible (does not go to completion)
• Some product is lost during transfer or purification
• Side reactions produce unwanted products
• Impurities in the reactants may interfere
How do you identify the limiting reactant in a reaction?
The limiting reactant is the one that is completely used up first, stopping the reaction.
To find it:
1. Calculate the moles of each reactant
2. Use the balanced equation ratio to see which runs out first
3. The one that runs out first is the limiting reactant
A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Determine its empirical formula.
1. Assume 100 g, so: 40.0 g C, 6.7 g H, 53.3 g O
2. Convert to moles:
C: 40.0/12 = 3.33 mol
H: 6.7/1 = 6.7 mol
O: 53.3/16 = 3.33 mol
3. Divide by smallest (3.33):
C: 1, H: 2, O: 1
4. Empirical formula: CH₂O
To find the molecular formula, divide the Mr by the empirical formula mass and multiply.
Key Questions: Calculations
What units are used to measure concentration?
Concentration can be measured in grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Mol/dm³ is also known as molarity (M).
What is a mole and what is the Avogadro constant?
A mole (mol) is the unit of amount of substance. One mole contains 6.02 x 10²³ particles (atoms, ions, molecules); this number is the Avogadro constant.
What is the molar gas volume at room temperature and pressure (r.t.p.)?
At room temperature and pressure (r.t.p.), the molar gas volume is taken as 24 dm³. This means one mole of any gas occupies 24 dm³ at r.t.p.
Tips to avoid common mistakes in Calculations
- ● Deploy the formula ΔH = Σ(bonds broken) - Σ(bonds formed), accounting for every single bond in every molecule, multiplied by the number of moles according to the balanced equation.
- ● For gas volume calculations, first determine the moles of EACH gaseous product, THEN multiply the TOTAL moles of gas by 24.0 dm3/mol.
- ● Before any stoichiometry calculation, rewrite the balanced equation and double-check every coefficient to be sure you have the correct mole ratios.
- ● When finding formula mass, pay attention to the little numbers in the formula (subscripts) and always use relative ATOMIC mass, not atomic number.
- ● Figure out which experiment requires more reagent to reach its endpoint.
More topics in Unit 3 — Stoichiometry
Calculations sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Key terms covered in this Calculations deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
Related Chemistry guides
Long-read articles that go beyond the deck — cover the whole subject's common mistakes, high-yield content and revision pacing.
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