HSC Chemistry — Module 6
Titration — Flashcards & Quiz
Titration is a quantitative technique for determining the concentration of an acid or base, and HSC Chemistry Module 6 tests both the procedure and the calculation. You need to distinguish equivalence point from endpoint, choose an appropriate indicator based on the pH curve, and perform accurate mole-ratio calculations — strong/strong, weak/strong, strong/weak — each with a characteristic pH curve shape you should be able to sketch.
Key Points
- Titration determines the concentration of an unknown by reacting it with a known-concentration standard until a stoichiometric endpoint.
- Equivalence point = moles of acid equal moles of base (stoichiometrically complete); endpoint = observed indicator colour change (which should closely match).
- Strong acid + strong base: sharp vertical pH jump around pH 7; almost any indicator works (phenolphthalein or methyl orange).
- Weak acid + strong base: equivalence point > 7 (conjugate base is weakly basic) — use phenolphthalein (colour change around pH 8-10).
- Weak base + strong acid: equivalence point < 7; use methyl orange (colour change around pH 3-5).
- Calculation: c₁V₁ / n₁ = c₂V₂ / n₂ where n is the stoichiometric coefficient from the balanced equation.
Common Mistakes to Avoid
- Confusing equivalence point (stoichiometric) with endpoint (observed colour change).
- Using 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 incorrectly — always read at the meniscus from below.
- Forgetting that the concentration of the standard must be known precisely.
Exam Strategy
HSC Module 6 titration questions ask you to (1) calculate unknown concentration, (2) choose an indicator, or (3) interpret a pH curve. Method: (1) write the balanced equation, (2) use c₁V₁/n₁ = c₂V₂/n₂ to find the unknown, (3) for indicators, identify the equivalence pH and choose an indicator whose colour change happens in that range.
Sample Flashcards
Q1: Describe the titration process.
Titration is a quantitative technique to determine the concentration of an unknown solution by reacting it with a solution of known concentration (standard solution). A burette delivers the titrant dropwise to the analyte until the equivalence point (stoichiometric amounts react). An indicator signals the end point.
Q2: Describe the shape of a strong acid-strong base titration curve.
Initial pH is low (~1-2, acidic). pH rises slowly as base is added. Near the equivalence point, pH rises steeply (sharp jump from ~3 to ~11). At the equivalence point, pH = 7 (neutral salt). After equivalence, pH levels off at high values (~12-13).
Q3: What is a primary standard and why is it used?
A primary standard is a pure, stable substance of known composition used to prepare a solution of accurately known concentration (standard solution). Properties: high purity, stable (doesn't absorb water or react with air), high molar mass (reduces weighing error), reacts completely and stoichiometrically.
Q4: What is a back titration and when is it used?
A back titration is used when the analyte cannot be directly titrated (insoluble, slow to react, or volatile). Excess of a known reagent is added to react with the analyte, then the remaining excess is titrated with a standard solution. The amount of analyte is found by difference.
Sample Quiz Questions
Q1: Phenolphthalein is a suitable indicator for a weak acid-strong base titration.
Answer: TRUE
Weak acid + strong base has an equivalence point at pH > 7 (~8-9). Phenolphthalein changes colour at pH 8.2-10.0, which matches this range.
Q2: The equivalence point is when the indicator changes colour.
Answer: FALSE
The equivalence point is when stoichiometric amounts of acid and base have reacted. The END POINT is when the indicator changes colour. They are close but not identical.
Q3: A primary standard must be of high purity and stable in air.
Answer: TRUE
Primary standards must be pure, stable (not hygroscopic, not reactive with air), have high molar mass, and react completely and stoichiometrically.
Revision Tip
Titration calculations are procedural — drill a Revizi flashcard deck of 8-10 titrations covering strong/weak acid with strong/weak base combinations.
Related Concepts
Last updated: March 2026 · 4 flashcards · 4 quiz questions