WACE Chemistry — Unit 3
Titration — Flashcards & Quiz
Titration is a quantitative technique for determining an unknown concentration by reacting it with a standard solution of known concentration. WACE Chemistry Year 12 Unit 3 tests both the experimental procedure and the calculation. You need to distinguish equivalence point (stoichiometric) from endpoint (indicator colour change), choose the right indicator based on the pH curve, and perform accurate mole-ratio calculations.
Key Points
- Equivalence point: moles of acid = moles of base (stoichiometrically); no excess of either.
- Endpoint: when the indicator changes colour — chosen so it matches the equivalence point as closely as possible.
- Strong acid + strong base: sharp pH jump around pH 7; almost any indicator works (phenolphthalein or methyl orange).
- Weak acid + strong base: equivalence point pH > 7 (conjugate base is weakly basic); use phenolphthalein (pH 8–10 range).
- Weak base + strong acid: equivalence point pH < 7; use methyl orange (pH 3–5 range).
- Calculation: c₁V₁/n₁ = c₂V₂/n₂ where n is the stoichiometric coefficient. Solve for the unknown concentration.
Common Mistakes to Avoid
- Confusing equivalence point (stoichiometric) with endpoint (observed indicator colour change).
- Choosing the wrong indicator — phenolphthalein for weak acid + strong base, methyl orange for strong acid + weak base.
- Not balancing the neutralisation equation before applying mole ratios.
- Reading the burette at the wrong meniscus position — always read at the bottom of the meniscus at eye level.
- Forgetting that the standard must have precisely known concentration.
Exam Strategy
SCSA Unit 3 titration questions give you the standard and titre volumes and ask you to calculate the unknown concentration. Method: (1) write the balanced equation, (2) calculate moles of the standard, (3) use the mole ratio from the equation, (4) divide by the unknown volume to get concentration. Always show working step by step and state final units (mol L⁻¹).
Sample Flashcards
Q1: Describe the key features of a strong acid–strong base titration curve.
The curve has three regions: 1) Gradual pH change far from equivalence point. 2) A steep vertical section at the equivalence point where pH changes rapidly. 3) Gradual change beyond equivalence. Equivalence point pH = 7 (neutral salt formed). The steep region spans roughly pH 3–11, so almost any indicator works.
Q2: How does a weak acid–strong base titration curve differ from a strong acid–strong base curve?
Key differences: 1) Starting pH is higher (weak acid is less dissociated). 2) There is a buffer region before equivalence where pH changes slowly. 3) Equivalence point pH > 7 (the conjugate base of the weak acid is basic). 4) The steep region is shorter (roughly pH 7–11). An indicator with a colour change above pH 7 is needed (e.g. phenolphthalein).
Sample Quiz Questions
Q1: The equivalence point of a strong acid–strong base titration is always at pH 7.
Answer: TRUE
At equivalence, only the neutral salt and water are present. Neither the cation nor anion hydrolyses, so pH = 7 at 25°C.
Q2: The equivalence point of a weak acid–strong base titration is below pH 7.
Answer: FALSE
The equivalence point is ABOVE pH 7 because the conjugate base of the weak acid hydrolyses to produce OH⁻ ions, making the solution basic.
Q3: At the half-equivalence point of a weak acid–strong base titration, pH equals pKa.
Answer: TRUE
At half-equivalence, exactly half the weak acid has been neutralised so [HA] = [A⁻]. By the Henderson-Hasselbalch equation, pH = pKa + log(1) = pKa.
Revision Tip
Titration calculations are procedural — drill a Revizi flashcard deck with 10+ titrations covering strong/weak acid and strong/weak base combinations.
Last updated: March 2026 · 2 flashcards · 3 quiz questions