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Revision guide · A-Level

A-Level Chemistry acids, bases and buffers: pH, Ka and titration curves

A Brønsted-Lowry acid donates a proton and a base accepts one. pH = -log[H+], so each pH unit is a tenfold change in hydrogen ion concentration. Strong acids dissociate fully; weak acids only partly, described by Ka. A buffer, a weak acid with its conjugate base (or a weak base with its conjugate acid), resists changes in pH when small amounts of acid or base are added. In Cambridge 9701 the qualitative ideas and titration curves are AS (topic 7.2) and pH, Ka, Kw and buffer calculations are A Level (topic 25.1). In Edexcel IAL all of this is Unit 4 Topic 14, with Core Practical 11 finding Ka. Marks are lost on the weak acid approximations and on logs.

Facts checked:

At a glance

Cambridge 9701
Topic 7.2 (AS); topic 25.1 (A Level)
Edexcel IAL
Unit 4 Topic 14 (WCH14)
Core practical
IAL CP11: finding Ka for a weak acid
At 298 K
Kw = 1.00 × 10^-14 mol^2 dm^-6

Where acids and buffers sit in each specification

Board and unitContent
Cambridge 9701 topic 7.2 (AS)Brønsted-Lowry theory, strong and weak acids and bases, pH of solutions (qualitative), neutralisation and salt formation, pH titration curves, choosing indicators (pKa values not used)
Cambridge 9701 topic 25.1 (A Level)Conjugate pairs; pH, Ka, pKa and Kw calculations (Kb not tested); pH of strong acids, strong alkalis and weak acids; buffers, how they work, the HCO3- system in blood, buffer pH calculations; Ksp and the common ion effect
Edexcel IAL Unit 4 Topic 14Brønsted-Lowry acids and conjugate pairs; pH; strong and weak acids; Ka and pH of weak acids (no quadratics); Kw, pKa, pKw; Ka from experimental data; titration curves for monoprotic and diprotic acids; indicators; buffers and their calculations; Ka from half-neutralisation; buffers in blood and food; Core Practical 11

The key ideas

pH = -log10[H+] and [H+] = 10^(-pH). For a strong monoprotic acid, [H+] equals the acid concentration. For a strong base, use Kw = [H+][OH-] = 1.00 × 10^-14 mol^2 dm^-6 at 298 K to find [H+], then pH.

For a weak acid HA ⇌ H+ + A-, Ka = [H+][A-]/[HA]. Assume [H+] = [A-] (the acid is the only source of ions) and that the equilibrium [HA] is approximately the starting concentration (very little dissociates). Then [H+] = sqrt(Ka × [HA]). pKa = -log Ka; a smaller pKa means a stronger acid.

A buffer contains a weak acid and its conjugate base in significant amounts. Added H+ reacts with A- to form HA; added OH- reacts with HA to form A- and water, so [H+] barely changes. Its pH is found from Ka = [H+][A-]/[HA], so [H+] = Ka × [HA]/[A-]. At half-neutralisation of a weak acid, [HA] = [A-], so pH = pKa, which is how Ka is found from a titration curve.

Worked example 1: pH of 0.100 mol dm^-3 ethanoic acid (Ka = 1.74 × 10^-5 mol dm^-3)

  1. Ka = [H+]^2/[CH3COOH], assuming [H+] = [CH3COO-] and [CH3COOH] ≈ 0.100.
  2. [H+]^2 = 1.74 × 10^-5 × 0.100 = 1.74 × 10^-6.
  3. [H+] = 1.32 × 10^-3 mol dm^-3.
  4. pH = -log(1.32 × 10^-3) = 2.88.
  5. Check the assumption: only about 1.3% of the acid dissociated, so treating [HA] as 0.100 is reasonable.

Worked example 2: pH of a buffer

  1. A buffer contains 0.10 mol dm^-3 ethanoic acid and 0.20 mol dm^-3 sodium ethanoate. Ka = 1.74 × 10^-5 mol dm^-3.
  2. [H+] = Ka × [HA]/[A-] = 1.74 × 10^-5 × 0.10/0.20 = 8.7 × 10^-6 mol dm^-3.
  3. pH = -log(8.7 × 10^-6) = 5.06.
  4. This makes sense: there is more conjugate base than acid, so the pH is above pKa (4.76).

Worked example 3: pH of 0.050 mol dm^-3 sodium hydroxide at 298 K

  1. NaOH is a strong base, so [OH-] = 0.050 mol dm^-3.
  2. [H+] = Kw/[OH-] = 1.00 × 10^-14 ÷ 0.050 = 2.0 × 10^-13 mol dm^-3.
  3. pH = -log(2.0 × 10^-13) = 12.70.

Common mistakes that cost marks

  • Using the strong acid method for a weak acid, so the pH is far too low.
  • Forgetting that a diprotic strong acid such as H2SO4 is usually treated as giving 2 mol of H+ per mole, if the question says so.
  • Not stating the two weak-acid assumptions when asked.
  • Calculating pH of a strong base directly from [OH-], giving pOH instead of pH.
  • Explaining buffers without equations; both boards ask for equations to show how pH is controlled.
  • Choosing an indicator whose colour change range is not within the vertical part of the titration curve.

Exam technique and how a tutor helps

Write the expression first, then substitute, then give pH to 2 decimal places, as pH values usually are. Know your calculator's log and 10^x keys well, because most errors in this topic are keystrokes rather than chemistry. For titration curves, know the shapes for strong-strong, strong-weak, weak-strong and weak-weak combinations, the starting pH, the equivalence point and the buffer region.

In buffer explanations, name both species and write both equations: A- + H+ → HA for added acid, and HA + OH- → A- + H2O for added alkali. Then say why the pH changes only slightly: the ratio [HA]/[A-] hardly changes because both are present in large amounts.

In one-to-one lessons a tutor works through a mixed set of pH problems with the student, deciding first which type each one is (strong acid, weak acid, strong base, buffer, after mixing), because choosing the right method is the real skill. The tutor checks calculator use on screen and builds a one-page method sheet with the student.

Self-check: can you do these?

  • pH of 0.0100 mol dm^-3 HCl. (Answer: 2.00)
  • [H+] of a solution with pH 3.40. (Answer: 3.98 × 10^-4 mol dm^-3)
  • What is the pH at half-neutralisation of a weak acid with pKa 4.20? (Answer: 4.20)
  • Which ion in blood acts as a buffer with carbonic acid? (Answer: HCO3-)
  • Why is the pH of a weak acid higher than a strong acid of the same concentration? (Answer: it only partly dissociates, so [H+] is lower)

Common questions

Are pH calculations AS or A Level?

In Cambridge 9701, AS topic 7.2 covers acids qualitatively and titration curves, and the pH, Ka, Kw and buffer calculations are in A Level topic 25.1. In Edexcel IAL they are all in Unit 4 Topic 14.

Will I need to solve a quadratic for weak acid pH?

Edexcel IAL states that students will not be expected to solve quadratic equations, so the standard approximations are used. For Cambridge 9701, use the same approximations and check past mark schemes to see how answers are credited.

Is Kb examined?

Cambridge 9701 states that Kb and the equation Kw = Ka × Kb will not be tested. Edexcel IAL focuses on Ka, pKa, Kw and pKw.

What is Core Practical 11?

In Edexcel IAL, Core Practical 11 finds Ka for a weak acid, typically by measuring the pH at half-neutralisation, where pH = pKa.

How much do LiveTutor A-Level Chemistry lessons cost?

Every lesson is one to one, online and 60 minutes, at one flat rate of $15 a lesson for every subject and level. Families choose a weekly plan of 1 to 5 lessons billed monthly, and the first lesson is a free trial.

Sources

Dates and figures on this page come from these official and published sources. Always confirm deadlines on the official page before acting on them.

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