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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- understand and use the terms conjugate acid and conjugate base
- define conjugate acid–base pairs, identifying such pairs in reactions
- define mathematically the terms pH, Ka, pKa and Kw and use them in calculations (Kb and the equation Kw = Ka × Kb will not be tested)
- calculate [H+(aq)] and pH values for: (a) strong acids (b) strong alkalis (c) weak acids
- calculate the pH of buffer solutions, given appropriate data
- understand and use the term solubility product, Ksp
- write an expression for Ksp
- calculate Ksp from concentrations and vice versa 10 (a) understand and use the common ion effect to explain the different solubility of a compound in a solution containing 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-.
- › Remember that Kp is only affected by temperature; increasing temperature in an exothermic reaction will decrease Kp.
- › Use an 'Initial, Change, Equilibrium' (ICE) table to track mole quantities and ensure the total moles include all species present.
- › Include all species in the Kc expression for non-aqueous equilibria; if 1.5 mol of ester forms, 1.5 mol of water must also exist.
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?
Ka is the acid dissociation constant, measuring the strength of an acid. pKa = -log₁₀Ka. A smaller pKa value indicates a stronger acid, meaning it dissociates more readily 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
Browse every 9701 flashcard topic by syllabus area.
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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
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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