CCEA A Level Chemistry

CCEA A Level Chemistry 4.5 Acid-Base Equilibria Exam Questions

Free CCEA A Level Chemistry revision for 4.5 Acid-Base Equilibria: what you need to know, common mistakes and exam-style questions with answers. Covers Bronsted-Lowry theory, Kw, Ka and pKa, pH calculations, buffers, titration curves, indicators and salt solutions.

Practise 4.5 Acid-Base Equilibria with past papers

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About CCEA A Level Chemistry 4.5 Acid-Base Equilibria

Section 4.5 of Unit A2 1 is one of the most calculation-heavy parts of the CCEA course. You need Bronsted-Lowry theory and conjugate acid-base pairs, and to define and use Kw, Ka, pH, pKw and pKa with units. You must calculate the pH of strong acids, strong bases and weak acids, explain how a buffer works, and calculate the pH of a buffer made by partly neutralising a weak monobasic acid with sodium hydroxide. You also need to describe how titration curves are obtained, use them to choose an indicator, and predict whether a salt solution is acidic, alkaline or neutral. This builds on AS titrations (1.9) and equilibrium (4.4).

What you need to know

  • Bronsted-Lowry An acid is a proton donor and a base is a proton acceptor. In CH3COOH + H2O in equilibrium with CH3COO- + H3O+, CH3COOH and CH3COO- are a conjugate pair.
  • pH and Kw pH = -log[H+]. Kw = [H+][OH-] = 1.00 x 10^-14 mol2 dm-6 at 298 K. For a strong base, [H+] = Kw / [OH-].
  • Weak acids Ka = [H+][A-] / [HA], units mol dm-3; pKa = -log Ka. Assuming [H+] = [A-] and [HA] at equilibrium is approximately the initial concentration, [H+] = square root of (Ka x [HA]).
  • Buffers A buffer resists changes in pH when small amounts of acid or alkali are added. In an ethanoic acid / ethanoate buffer, added H+ combines with CH3COO- and added OH- reacts with CH3COOH. [H+] = Ka x [HA] / [A-].
  • Titration curves Measure pH with a calibrated pH meter as base is added from a burette. Strong acid/strong base has a vertical section from about pH 3 to 11; weak acid/strong base has an equivalence point above 7; strong acid/weak base below 7. Choose an indicator whose range lies within the vertical section.
  • Salts A salt of a strong acid and weak base (NH4Cl) is acidic; a salt of a weak acid and strong base (CH3COONa) is alkaline; a salt of a strong acid and strong base (NaCl) is neutral.

Common mistakes in 4.5 Acid-Base Equilibria

  • Using the initial concentration of a strong base as [H+]. Use Kw to find [H+] from [OH-].
  • Forgetting to account for partial neutralisation in buffer calculations. Subtract the moles of OH- added from HA and use them as moles of A-.
  • Giving pH to too many or too few decimal places. Two decimal places is usually appropriate.
  • Saying a buffer keeps pH constant. It minimises changes when small amounts of acid or alkali are added.
  • Choosing phenolphthalein for a weak base with a strong acid. The equivalence point is acidic, so methyl orange is suitable.

Try three 4.5 Acid-Base Equilibria exam-style questions

1. Calculate the pH of 0.100 mol dm-3 ethanoic acid (Ka = 1.74 x 10^-5 mol dm-3). (2 marks)

Show the mark scheme answer

[H+] = square root of (1.74 x 10^-5 x 0.100) = 1.32 x 10^-3; pH = 2.88.

2. Calculate the pH of 0.0200 mol dm-3 sodium hydroxide at 298 K. (2 marks)

Show the mark scheme answer

[H+] = 1.00 x 10^-14 / 0.0200 = 5.00 x 10^-13; pH = 12.30.

3. 50.0 cm3 of 0.200 mol dm-3 ethanoic acid is mixed with 25.0 cm3 of 0.100 mol dm-3 NaOH. Calculate the pH of the buffer (Ka = 1.74 x 10^-5). (4 marks)

Show the mark scheme answer

Moles HA = 0.0100, moles OH- = 0.00250; HA left = 0.00750, A- = 0.00250; [H+] = 1.74 x 10^-5 x 0.00750 / 0.00250 = 5.22 x 10^-5; pH = 4.28.

4.5 Acid-Base Equilibria FAQs

What does CCEA A Level Chemistry 4.5 Acid-Base Equilibria cover?

It covers Bronsted-Lowry theory, Kw, Ka and pKa, pH calculations, buffers, titration curves, indicators and the pH of salt solutions.

Which unit is 4.5 Acid-Base Equilibria examined in?

It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.

Which pH calculations do I need to practise?

Strong acids, strong bases using Kw, weak acids using Ka, and buffers made by adding sodium hydroxide to excess weak acid.