Acids, Bases and Neutralisation: Titration Technique and Calculations

This guide covers 2.1.4, Acids, from Module 2, Foundations in chemistry, of OCR A Level Chemistry A H432. It follows Amount of substance (2.1.3) and precedes 2.1.5 Redox, applying the mole calculations from 2.1.3 directly to acid-base practical chemistry.

Common acids and alkalis

You need to know the formulae of the common acids – HCl, H₂SO₄, HNO₃ and CH₃COOH – and the common alkalis – NaOH, KOH and NH₃ – and be able to explain that acids release H⁺ ions in aqueous solution while alkalis release OH⁻ ions in aqueous solution. This is the definitional foundation the rest of the subtopic builds on: every reaction and calculation that follows treats acids and alkalis in terms of these ions.

Strong versus weak acids

You need to give a qualitative explanation of strong and weak acids in terms of relative dissociation. A strong acid dissociates (ionises) almost completely in aqueous solution, releasing close to the maximum possible concentration of H⁺ ions, while a weak acid only partially dissociates, with most of the acid remaining as undissociated molecules in equilibrium with a smaller concentration of H⁺ ions. Note that this subtopic requires only a qualitative explanation – the quantitative treatment of weak acid equilibria (using Ka and pH calculations) belongs to later Physical Chemistry content, not here.

Neutralisation

You need to understand neutralisation as the reaction of H⁺ and OH⁻ to form H₂O, and as the reaction of acids with bases – including carbonates, metal oxides and alkalis (water-soluble bases) – to form salts, with full equations required. This distinguishes an important vocabulary point: “base” is the broader category (any species that neutralises an acid, including insoluble metal oxides and carbonates), while “alkali” specifically means a base that is soluble in water and therefore releases OH⁻ ions directly into solution. Every alkali is a base, but not every base is an alkali.

Acid-base titration technique

You need to know the techniques and procedures used when preparing a standard solution of required concentration and carrying out acid-base titrations – this is Practical Skill PAG2 in the specification, and links to the “How Science Works” strand covering opportunities for experimental and investigative work. Preparing a standard solution involves accurately weighing a solid solute, dissolving it, and making the solution up to a precise volume in a volumetric flask. Carrying out a titration involves using a burette to add a solution of known concentration to a measured volume of the solution being analysed, typically with an indicator to signal the neutralisation endpoint.

Titration calculations

You need to be able to carry out structured and non-structured titration calculations, based on experimental results, for both familiar and unfamiliar acids and bases. This draws directly on the mole-calculation skills from 2.1.3: given a titre volume and known concentration of one solution, you use the mole ratio from the balanced equation to find the moles, then concentration, of the unknown solution. “Non-structured” calculations expect you to plan the full working method yourself, rather than following a series of guided steps – a skill worth practising deliberately, since guided structured questions can mask gaps in your ability to set up a calculation from scratch.

A worked example

25.0 cm³ of a sodium hydroxide solution of unknown concentration is exactly neutralised by 22.5 cm³ of 0.100 mol dm⁻³ hydrochloric acid. The equation is NaOH + HCl → NaCl + H₂O, a 1:1 mole ratio. Moles of HCl used: 0.0225 dm³ × 0.100 mol dm⁻³ = 0.00225 mol. Since the ratio is 1:1, moles of NaOH = 0.00225 mol. Concentration of NaOH = 0.00225 mol ÷ 0.0250 dm³ = 0.0900 mol dm⁻³. Working through the mole ratio explicitly, rather than assuming it is always 1:1, is essential preparation for questions involving acids such as H₂SO₄ that react in different ratios with a given base.

Why acid-base chemistry connects across the syllabus

This subtopic is not an isolated block of practical chemistry. The formula-writing and equation-balancing skills from 2.1.2 are applied directly when writing the full equations for neutralisation reactions here, and the mole-based calculation method from 2.1.3 is the mechanism every titration calculation in this subtopic depends on. The qualitative treatment of strong and weak acid dissociation introduced here is also revisited quantitatively later in the course, once equilibrium constants are covered in Physical Chemistry – so a solid qualitative grasp of what “partial dissociation” actually means now will make the later quantitative treatment considerably easier to follow.

How to approach it

Keep the acid/alkali/base distinction precise: alkali is a subset of base, and mixing the terms up in an explanation can cost marks even when the chemistry is otherwise correct. Practise the full mole-ratio-based titration calculation method on a range of acid-base combinations, not only 1:1 reactions like NaOH with HCl, since the specification explicitly includes reactions with carbonates and other bases that do not neutralise in a simple 1:1 ratio. When describing strong versus weak acids, use the language of “dissociation” or “ionisation” and relative extent, rather than describing weak acids as simply “less acidic” – concentration and strength are different properties, and a dilute strong acid can have a lower H⁺ concentration than a concentrated weak acid despite being fundamentally the “stronger” acid. For the practical technique itself, be ready to describe the full standard-solution and titration procedure in words, since exam questions frequently ask candidates to describe or evaluate practical method rather than only calculate a result.

Official syllabus

OCR, Cambridge OCR Level 3 Advanced GCE in Chemistry A (H432) specification, Version 3.1, May 2026: official specification PDF, section 2.1.4 “Acids”. Verified 2026-09-02.