Quantitative chemistry: GCSE Chemistry
Conservation of mass, relative formula mass, moles, concentration, yield and atom economy
The short version
Use M_r to convert between mass and amount of substance. Concentration is how much solute is dissolved in a volume of solution. Yield compares what you got with the maximum possible.
Key idea
Ask: did a gas leave, or did a gas from the air join?
Grade 2–3: If a gas puffs away, the stuff in the dish weighs less. The gas still has mass.
Grade 5–6: I can say closed vs open and name CO2 leaving or O2 joining.
Grade 9: I use the balanced equation as a counting version of conservation of mass. I never write atoms are destroyed.
Go a bit deeper
Foundation-friendly, still appears on Higher
Grade 5–6: I can do % by mass and I know 3 × sulfate means 12 oxygen, not 4.
Grade 9: From 2Mg + O2 → 2MgO, 48 g Mg makes 80 g MgO, so 12 g Mg makes 20 g MgO. Higher will go via moles instead.
Question: Calculate M_r of Al2(SO4)3 (Al=27, S=32, O=16) and % by mass of Fe in Fe2O3 (Fe=56, O=16). [4 marks]
Worked example
Worked example
Grade 2–3: If a gas puffs away, the stuff in the dish weighs less. The gas still has mass. Grade 5–6: I can say closed vs open and name CO2 leaving or O2 joining. Grade 9: I use the balanced equation as a counting version of conservation of mass. I never write atoms are destroyed.
Lessons in this topic
- Conservation of mass
- Relative formula mass
- Moles (Higher)
- Concentration
- Percentage yield (Combined)
- Atom economy (Higher)
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Write: moles = mass / M_r. Concentration in g/dm³ = mass / volume. Percentage yield = (actual / theoretical) × 100. Atom economy is Higher.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
In a closed system, mass is conserved because
Heating CaCO₃ in an open crucible loses mass because
M_r of H₂SO₄ (H=1, S=32, O=16) is
% by mass of H in H₂O (H=1, O=16) is
Common mistakes
- Students write "mass is destroyed". It is not — it left as a gas, or a gas from the air was not counted.
- Al2(SO4)3 has 3 sulfate groups: 3 sulfur and 12 oxygen, not 4 oxygen.
- Use grams, not kilograms. Convert 250 mg to 0.250 g before you divide. 1 mole is 6.02 × 10^23 particles — do not treat that number as a mass.
- Using 250 instead of 0.250 gives an answer 1000 times too small. Convert the volume first. Do not mix g/dm^3 with mol/dm^3.
Answers and working
Frequently asked questions
Is GCSE Quantitative chemistry on Combined Science and Triple?
Quantitative chemistry is on Combined Science and on Triple Chemistry. Progress is shared in the ReviseEasy app.
Which exam board is this Quantitative chemistry page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Quantitative chemistry Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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