Acids and bases
Cambridge A-Level Chemistry (9701) · Unit 25: Equilibria · 10 flashcards
Acids and bases is topic 25.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 25 — Equilibria , alongside Partition coefficients. In one line: A conjugate acid is a species formed when a base accepts a proton (H⁺). A conjugate base is a species formed when an acid donates a proton (H⁺).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 10 flashcards — 6 definitions, 2 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 6 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The terms 'conjugate acid' and 'conjugate base'
A conjugate acid is a species formed when a base accepts a proton (H⁺). A conjugate base is a species formed when an acid donates a proton (H⁺).
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand what conjugate acids and conjugate bases are and use these terms.
- Define a conjugate acid-base pair and pick out such pairs in reactions.
- Define pH, Kₐ, pKₐ and Kw mathematically and apply these definitions in calculations; neither Kb nor Kw = Kₐ × Kb is tested.
- Calculate the pH and the [H⁺(aq)] of solutions of: (a) a strong acid; (b) a strong alkali; (c) a weak acid.
- (a) Define a buffer solution. (b) Explain how to make a buffer solution. (c) Explain, with chemical equations, how buffer solutions keep pH under control. (d) Describe and explain what buffer solutions are used for, including how HCO₃⁻ controls the pH of blood.
- Calculate the pH of a buffer solution from suitable data.
- Understand the term solubility product, Kₛₚ, and use it.
- Write the Kₛₚ expression for a given compound.
- Calculate Kₛₚ from concentrations, and calculate concentrations from a given Kₛₚ.
- (a) Use the common ion effect to explain why a compound dissolves to a different extent in a solution that already contains one of its ions, and show you understand the idea. (b) Do calculations that combine Kₛₚ values with the concentration of a common ion.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Acids and bases
- › Identify the conjugate base by removing one proton (H+) from the Bronsted-Lowry acid; for H2S, the conjugate base is HS-.
Define the terms 'conjugate acid' and 'conjugate base'.
A conjugate acid is a species formed when a base accepts a proton (H⁺). A conjugate base is a species formed when an acid donates a proton (H⁺).
Identify the conjugate acid-base pairs in the following reaction: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
The conjugate acid-base pairs are: NH₃ (base) and NH₄⁺ (conjugate acid), and H₂O (acid) and OH⁻ (conjugate base).
Define pH mathematically and explain its significance.
pH is defined as -log₁₀[H⁺(aq)], where [H⁺(aq)] is the concentration of hydrogen ions in mol/L (or mol/dm³). pH indicates the acidity or alkalinity of a solution; lower pH values indicate higher acidity.
Define Ka and pKa mathematically. What does a smaller pKa value indicate?
For a weak acid HA ⇌ H⁺ + A⁻, Ka = [H⁺(aq)][A⁻(aq)] / [HA(aq)], the acid dissociation constant (units mol dm⁻³). pKa = −log₁₀Ka. A smaller pKa means a larger Ka, so the acid is stronger: it dissociates to a greater extent in solution.
Define Kw mathematically and state its value at 298K.
Kw is the ionic product of water, defined as Kw = [H⁺(aq)][OH⁻(aq)]. At 298K (25°C), Kw = 1.0 x 10⁻¹⁴ mol² dm⁻⁶.
A solution of a strong acid, HCl, has a concentration of 0.01 mol/L. Calculate the pH of this solution.
Since HCl is a strong acid, it completely dissociates. Therefore, [H⁺(aq)] = 0.01 mol/L. pH = -log₁₀(0.01) = 2.
A solution of a strong alkali, NaOH, has a concentration of 0.005 mol/L. Calculate the pH of this solution at 298K.
First, calculate [OH⁻(aq)] = 0.005 mol/L. Then, calculate [H⁺(aq)] using Kw = [H⁺(aq)][OH⁻(aq)], so [H⁺(aq)] = Kw/[OH⁻(aq)] = (1.0 x 10⁻¹⁴) / 0.005 = 2 x 10⁻¹² mol/L. Finally, pH = -log₁₀(2 x 10⁻¹²) = 11.70.
Define the term 'buffer solution' and explain its function.
A buffer solution resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid).
Write the expression for the solubility product, Ksp, of silver chloride (AgCl).
The dissolution of silver chloride is represented as: AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq). The solubility product expression is: Ksp = [Ag⁺(aq)][Cl⁻(aq)].
Explain the common ion effect and how it affects solubility.
The common ion effect is the decrease in solubility of a sparingly soluble salt when a soluble salt containing a common ion is added to the solution. The added common ion shifts the equilibrium of the sparingly soluble salt towards precipitation, decreasing its solubility.
More Chemistry flashcards
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Acids and bases sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Acids and bases deck
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