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Chemistry · 4. Acids, bases and pH scale

Acids, bases, pH scale and titrations

Explore proton donor-acceptor behavior, calculate pH, and quantify solution concentrations accurately via volumetric titration.

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

Driving question

How can we determine the precise concentration of an unknown acid or alkali using volumetric titration?

Curriculum coverage

Vietnam Science 8, Chemistry 11; IGCSE Acids, bases and salts; A-Level Ionic equilibria.

Model scope and limitations

pH = -log[H⁺] applies accurately to dilute solutions (< 1 M); concentrated solutions require ion activity models.

Learning objectives

  • Define acids and bases using Arrhenius and Brønsted-Lowry theories.
  • Calculate pH from [H⁺] or [OH⁻] and understand the logarithmic scale.
  • Perform dropwise titration, determine equivalence points and select valid indicators.

Prerequisite knowledge

  • Molar concentration and net ionic equations.
  • H⁺ and OH⁻ ions in aqueous solutions.

Core knowledge

Brønsted-Lowry Acid-Base Theory

An acid is a proton (H⁺) donor; a base is a proton acceptor. A conjugate acid-base pair differs by exactly one H⁺ ion.

The pH Scale and Water Autoionisation

At 25°C, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴. pH = -log[H⁺] and pH + pOH = 14. Each unit change on the logarithmic pH scale represents a 10-fold change in [H⁺].

Equivalence Point and Titration Inflection

At equivalence, moles of H⁺ exactly equal moles of OH⁻. An indicator changes sharply only when its transition range falls within the steep vertical inflection of the titration curve.

Worked example

Titrating 25.0 cm³ of unknown HCl against 0.10 M NaOH requires 20.0 cm³ NaOH at phenolphthalein endpoint. n_NaOH = 0.020 × 0.10 = 0.0020 mol = n_HCl. Thus c_HCl = 0.0020 / 0.025 = 0.080 mol/dm³.

Misconceptions to avoid

  • Equivalence is not always at pH 7.0; hydrolysis of conjugate weak acid/base salts shifts the equivalence pH.
  • Strong and weak acids of equal molarity require the identical volume of alkali for complete neutralisation.

Virtual experiment procedure: Acid-base titration and pH curve

  1. Rinse the burette with distilled water and prime it with standard NaOH titrant.
  2. Pipette an exact volume of acid into a conical flask and add 2-3 drops of indicator.
  3. Titrate dropwise with continuous swirling until the faintest persistent pink colour remains for 30 seconds.

Safety and cautions

  • Acids and alkalis are corrosive; wear safety goggles and rinse immediately under flowing water for 15 minutes if skin contact occurs.

Evidence to collect

  • Burette reading table with initial, final and titre volumes for at least three concordant runs (within ±0.1 cm³).
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Diagnostic check

1. A solution with [H⁺] = 1.0 × 10⁻³ M has what pH value?

  1. pH = 3
  2. pH = 11
  3. pH = 1
Show answer and explanation

Correct answer: pH = 3

pH = -log(10⁻³) = 3.

2. Under Brønsted-Lowry, in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, H₂O acts as a/an?

  1. Acid (proton donor)
  2. Base (proton acceptor)
  3. Catalyst
Show answer and explanation

Correct answer: Acid (proton donor)

H₂O donates a proton to NH₃, acting as a Brønsted-Lowry acid.

3. Which indicator is best for titrating weak acid CH₃COOH with strong base NaOH?

  1. Phenolphthalein
  2. Methyl Orange
  3. Litmus paper
Show answer and explanation

Correct answer: Phenolphthalein

The equivalence point is basic (pH ~ 8.5-9.0), perfectly matching phenolphthalein transition.

Application practice

1. Mixing 100 cm³ 0.1 M HCl with 100 cm³ 0.1 M NaOH results in what pH at 25°C?

  1. pH = 7
  2. pH = 1
  3. pH = 13
Show answer and explanation

Correct answer: pH = 7

Equal moles of strong acid and base neutralise completely to form neutral NaCl solution, pH = 7.

2. A solution at pH = 12 has what [OH⁻] concentration at 25°C?

  1. 0.01 M (10⁻² M)
  2. 10⁻¹² M
  3. 1.0 M
Show answer and explanation

Correct answer: 0.01 M (10⁻² M)

pOH = 14 - 12 = 2 ⇒ [OH⁻] = 10⁻² M = 0.01 mol/dm³.

3. Why does the conical flask not need to be dry before pipetting the acid sample?

  1. Water does not alter total acid moles in the flask
  2. Extra volume enhances visibility
  3. Water changes pH immediately
Show answer and explanation

Correct answer: Water does not alter total acid moles in the flask

The absolute number of moles is fixed by the pipette; additional water does not change the stoichiometric moles of acid.

Academic references

  1. Cambridge IGCSE Chemistry 0620 — official syllabus and resources
  2. Cambridge AS & A Level Chemistry 9701 — official syllabus
  3. OpenStax Chemistry 2e — peer-reviewed general chemistry
  4. IUPAC Color Books — authoritative chemical terminology and nomenclature
  5. 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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