25.1 Acids and Bases

2026 Syllabus Objectives

By the end of this topic, you should be able to:

Understand and use the terms conjugate acid and conjugate base
Define conjugate acid–base pairs and identify them in reactions
Define mathematically pH, Ka, pKa, and Kw, and use them in calculations
Calculate [H⁺(aq)] and pH for: (a) strong acids, (b) strong alkalis, (c) weak acids
(a) Define a buffer solution; (b) explain how one is made; (c) explain how buffers control pH using equations; (d) describe the role of HCO₃⁻ in controlling blood pH
Calculate the pH of buffer solutions
Understand and use the term solubility product, Ksp
Write an expression for Ksp
Calculate Ksp from concentrations, and vice versa
(a) Understand the common ion effect; (b) perform calculations using Ksp and common ion concentration

In A2 Chemistry, we use the Brønsted–Lowry definition of acids and bases:

A Brønsted–Lowry acid is a species (atom, molecule, or ion) that can donate (give away) a proton (H⁺ ion).
A Brønsted–Lowry base is a species that can accept (receive) a proton (H⁺ ion).

Think of it like a game of pass: the acid passes the H⁺ to the base.

When an acid donates a proton, it becomes a conjugate base — the species left behind after the H⁺ has been given away.

When a base accepts a proton, it becomes a conjugate acid — the species formed after the H⁺ has been received.

A conjugate acid–base pair is two species that differ from each other by exactly one H⁺ ion.

Think of it this way: the conjugate base is just the acid minus one H⁺, and the conjugate acid is just the base plus one H⁺.

\text{CH}_3\text{COOH (aq)} + \text{H}_2\text{O (l)} \rightleftharpoons \text{CH}_3\text{COO}^- \text{(aq)} + \text{H}_3\text{O}^+ \text{(aq)}