1. Which balances hydrogen burning to form water?
- H₂ + O₂ → H₂O
- H₂ + 2O₂ → H₂O
- 2H₂ + O₂ → 2H₂O
Check the answer and reasoning
2H₂ + O₂ → 2H₂O
There are four H atoms and two O atoms on each side.
CHEM / IGCSE
Formulae, equations and chemical amounts
Balanced equations conserve atoms and charge. Relative atomic and formula masses connect microscopic composition to measurable masses; a mole counts a fixed number of entities.
Learning objectives
- Explain formulae, equations and chemical amounts using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Balanced equations conserve atoms and charge. Relative atomic and formula masses connect microscopic composition to measurable masses; a mole counts a fixed number of entities.
Use coefficients as mole ratios, identify the limiting reagent and distinguish percentage yield from purity. Gas-volume and solution-concentration calculations require consistent units and stated conditions.
Equations and relationships
Amount in mol from mass and molar mass in matching units.
Worked example
How many moles are in 10 g of CaCO₃ with M=100 g/mol?
Follow the solution
Divide mass by molar mass: n=10/100=0.10 mol.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. How many moles are in 10 g of CaCO₃ with M=100 g/mol?
- 0.10 mol
- 10 mol
- 1000 mol
Check the answer and reasoning
0.10 mol
Divide mass by molar mass: n=10/100=0.10 mol.
3. Can a reaction make more atoms than it started with?
- Yes, in an exothermic reaction
- No
- Only with a catalyst
Check the answer and reasoning
No
Chemical reactions rearrange atoms; they do not create them.