Key Learning Objectives
- Apply the Law of Conservation of Mass: total mass and atom count of reactants equals products.
- Balance complex equations systematically using reactant and product element tallies.
- Write balanced ionic equations and identify spectator ions.
Topic Summary & Core Notes
Master the conservation of mass and stoichiometric balancing using the atom tally inventory method. Never change chemical formulas, sub-indices, or drop balancing marks.
Key Rules, Formulas & Definitions
Conservation of Mass
Total Mass of Reactants = Total Mass of Products
Balancing Rule
Only adjust stoichiometric COEFFICIENTS in front of formulas; never alter subscript numbers
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Hydrocarbon Combustion Balancing
Question: Balance the complete combustion equation for propane: C3H8 + O2 → CO2 + H2O.
Step-by-Step Solution:
- Step 1: Balance Carbons: 3 carbons on left → 3 CO2 on right.
- Step 2: Balance Hydrogens: 8 hydrogens on left → 4 H2O on right (4 × 2 = 8).
- Step 3: Count and balance Oxygens: Right side has (3 × 2) + 4 = 10 O atoms. Left side needs 5 O2: C3H8 + 5O2 → 3CO2 + 4H2O.
Final Answer: C3H8 + 5O2 → 3CO2 + 4H2O
Senior Examiner Insight: Balance C first, then H, and balance elementary O2 last.
Worked Example 2
Worked Example 2: Precipitation Ionic Equation
Question: Write the ionic equation for the reaction between aqueous silver nitrate and aqueous sodium chloride: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).
Step-by-Step Solution:
- Step 1: Split soluble ionic compounds: Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) → AgCl(s) + Na+(aq) + NO3-(aq).
- Step 2: Cancel spectator ions: Na+(aq) and NO3-(aq) appear unchanged on both sides.
- Step 3: Write net ionic equation: Ag+(aq) + Cl-(aq) → AgCl(s).
Final Answer: Ag+(aq) + Cl-(aq) → AgCl(s)
Senior Examiner Insight: State symbols are required in ionic equations. Insoluble precipitates must be labelled (s).
Common Pitfalls & Examiner Warnings
Common Mistake: Changing subscript numbers in formulas
Changing a subscript (e.g. changing H2O to H2O2) creates a completely different chemical compound.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What is the coefficient for O2 when balancing the equation for the formation of aluminium oxide: 4Al + ?O2 → 2Al2O3?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) 3
Detailed Explanation: Right side has 2 × 3 = 6 oxygen atoms. 6 ÷ 2 = 3 O2 molecules.
Mark Scheme & Scoring Points: 1 mark for 3.
Question 2 of 3
Which fundamental physical law dictates that chemical equations must be balanced?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Law of Conservation of Mass
Detailed Explanation: Atoms cannot be created or destroyed in a chemical reaction; they are merely rearranged.
Mark Scheme & Scoring Points: 1 mark for Law of Conservation of Mass.
Question 3 of 3
In the reaction: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s), which ions are spectator ions?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Sulfate ions (SO4^2-)
Detailed Explanation: Sulfate ions remain dissolved and unchanged in aqueous solution on both sides of the equation.
Mark Scheme & Scoring Points: 1 mark for Sulfate ions.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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