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Chemistry · 4. Acids, bases and salts

pH, neutralisation and acid–base titration

Determine an unknown concentration from reaction stoichiometry and the equivalence point.

45 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

How accurately can a colour change reveal an unknown concentration?

Curriculum coverage

Vietnam 2018 and Cambridge: pH, neutralisation, concentration and titration.

Model scope and limitations

pH values are educational estimates; temperature, ionic activity and weak species can shift equivalence away from pH 7.

Learning objectives

  • Distinguish equivalence point from indicator end point.
  • Calculate amount using n = C·V with V in litres.
  • Choose a suitable indicator and read a burette correctly.

Prerequisite knowledge

  • Moles, molar concentration and balanced equations
  • Meaning of pH

Core knowledge

Stoichiometry at equivalence

Acid and base amounts follow equation coefficients; the ratio is not always 1:1.

Volume conversion

In n = C·V, convert mL to L by dividing by 1000.

End point

The indicator changes near equivalence; choose its transition range within the steep pH change.

Worked example

25.00 mL HCl requires 20.00 mL of 0.1000 mol/L NaOH. For 1:1 stoichiometry, C(HCl) = 0.1000×20.00/25.00 = 0.08000 mol/L.

Misconceptions to avoid

  • Equivalence is stoichiometric; its pH is not always 7.
  • An indicator end point only approximates equivalence and can introduce indicator error.

Virtual experiment procedure: Acid–base titration

  1. Rinse the burette with titrant and record the initial reading at eye level.
  2. Add titrant quickly at first, then dropwise near the end point.
  3. Repeat until at least two concordant titres are obtained.

Safety and cautions

  • Acids and bases may be corrosive; wear eye protection and rinse splashes immediately.
  • Never pipette by mouth.

Evidence to collect

  • Initial/final burette readings, titres and concordant values.
  • A concentration calculation using the balanced equation and mean concordant titre.
Open the virtual experiment

Diagnostic check

1. In n = C·V, if C is in mol/L, which unit must V use?

  1. Litres
  2. Millilitres
  3. Centimetres
Show answer and explanation

Correct answer: Litres

Convert mL to L before multiplying by molar concentration.

2. What defines the equivalence point in a titration?

  1. Stoichiometrically equivalent amounts
  2. The solution always has pH 7
  3. The darkest possible colour
Show answer and explanation

Correct answer: Stoichiometrically equivalent amounts

Equivalence is defined by stoichiometry; its pH depends on the acid–base system.

3. How should uncertainty near the end point be reduced?

  1. Add dropwise and swirl
  2. Pour a large amount quickly
  3. Read the burette from above
Show answer and explanation

Correct answer: Add dropwise and swirl

Dropwise addition, swirling and eye-level reading reduce overshoot and parallax.

Application practice

1. How many moles are in 25.0 mL of 0.100 mol/L NaOH?

  1. 2.50×10⁻³ mol
  2. 2.50 mol
  3. 0.250 mol
Show answer and explanation

Correct answer: 2.50×10⁻³ mol

n = 0.100 × 0.0250 = 2.50×10⁻³ mol.

2. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. How much NaOH reacts with 0.010 mol H₂SO₄?

  1. 0.020 mol
  2. 0.010 mol
  3. 0.005 mol
Show answer and explanation

Correct answer: 0.020 mol

The coefficient ratio NaOH:H₂SO₄ is 2:1.

3. Why should equivalence pH not always be assumed to be 7?

  1. Weak acids or bases create hydrolysis equilibria
  2. A burette always raises pH
  3. pH is unrelated to chemistry
Show answer and explanation

Correct answer: Weak acids or bases create hydrolysis equilibria

Salts of weak acids/bases can hydrolyse, shifting equivalence pH from 7.

Academic references

  1. OpenStax Chemistry 2e — peer-reviewed general chemistry
  2. IUPAC Gold Book — authoritative chemical terminology
  3. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

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WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer