Common Acids and Their Formulae

Introduction

Chemistry laboratories use dozens of acids, but four turn up again and again: hydrochloric acid, sulfuric acid, nitric acid and ethanoic acid. Knowing their formulae is like knowing the names of the main characters in a story — almost every equation about acids, salts and neutralisation involves one of them. This page introduces each acid, the ions it releases in water and the salts it goes on to make.

Core explanation

Every acid releases hydrogen ions, H⁺(aq) , when it dissolves in water. What makes each acid different is the negative ion (anion) left behind. That anion ends up in the salt when the acid is neutralised.

Hydrochloric acid, HCl. A solution of hydrogen chloride gas in water. It is a strong acid, fully ionised:

HCl(aq) → H⁺(aq) + Cl⁻(aq)

The chloride ion, Cl⁻, gives salts called chlorides , such as sodium chloride. Hydrochloric acid is also found in the stomach, where it helps digestion.

Sulfuric acid, H₂SO₄. A strong acid with two hydrogens per formula unit:

H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)

The sulfate ion carries a 2− charge, so sulfuric acid can release two H⁺ ions for each formula unit. Its salts are sulfates , such as copper(II) sulfate. It is one of the most heavily manufactured chemicals in the world.

Nitric acid, HNO₃. A strong acid:

HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)

The nitrate ion, NO₃⁻, gives nitrates , such as potassium nitrate. Nitric acid is used on a large scale to make fertilisers.

Ethanoic acid, CH₃COOH. The acid in vinegar, and a weak acid. Only a small fraction of its molecules ionise, and the reaction is reversible:

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

Only the hydrogen on the COOH group is released; the three hydrogens on the CH₃ group stay attached. Its salts are ethanoates , such as sodium ethanoate.

Acid Formula Strong or weak Anion Salts called --- --- --- --- --- Hydrochloric HCl Strong Cl⁻ Chlorides Sulfuric H₂SO₄ Strong SO₄²⁻ Sulfates Nitric HNO₃ Strong NO₃⁻ Nitrates Ethanoic CH₃COOH Weak CH₃COO⁻ Ethanoates

Other acids you may meet. Phosphoric acid, H₃PO₄, gives phosphates and is found in some cola drinks. Citric acid in lemons is a weak organic acid. Carbonic acid, H₂CO₃, forms when carbon dioxide dissolves in water and makes fizzy drinks slightly acidic.

Step-by-step reasoning

To write the ionisation of an acid:

1. Write the formula of the acid. 2. Count the hydrogens that can be released (all of them for the mineral acids; only the COOH hydrogen for ethanoic acid). 3. Write that many H⁺ ions. 4. Write the anion that is left, with a charge equal to the number of H⁺ removed. 5. Use → for a strong acid and ⇌ for a weak acid.

Visual explanation

Imagine each acid molecule drawn as a small cluster in water. For HCl, HNO₃ and H₂SO₄, every cluster has split into separate H⁺ and anion dots. For ethanoic acid, most clusters stay whole, with only the odd H⁺ drifting free — a picture of a weak acid.

Real-world analogy

Think of the acids as a family who all share the same first name, "Hydrogen", but have different surnames: Chloride, Sulfate, Nitrate, Ethanoate. The shared first name gives them their family resemblance (acidity); the surname decides which salt they create.

Real-world example

Car batteries contain sulfuric acid, fertiliser factories use nitric acid, steelworks use hydrochloric acid to remove rust before coating steel, and kitchens use dilute ethanoic acid as vinegar. The same four acids appear across very different industries.

Why?

Why is sulfate SO₄²⁻ but nitrate NO₃⁻? Because the compound must be neutral overall. Sulfuric acid has two H⁺ ions, each 1+, so the anion left behind must carry 2−. Nitric acid has one H⁺, so its anion carries 1−.

Common misconception

"Ethanoic acid, CH₃COOH, has four hydrogens, so it releases four H⁺ ions." Only the hydrogen bonded to oxygen in the COOH group can be released. The hydrogens bonded to carbon are held too firmly, so ethanoic acid is monoprotic.

Worked example

Question: Write an equation for the ionisation of nitric acid and name the type of salt it forms.

Reasoning: HNO₃ contains one releasable hydrogen. Removing it as H⁺ leaves NO₃⁻ with a 1− charge. Nitric acid is strong, so use a single arrow.

Answer: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq); it forms nitrates.

Quick check

1. What is the formula and charge of the ion that sulfuric acid gives to its salts? Answer: The sulfate ion, SO₄²⁻.

Exam focus

Learn the four formulae and the salt names by heart: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate, ethanoic → ethanoate. The most common error is a wrong ion charge, so check that sulfate always carries 2− and nitrate and chloride carry 1−, and remember that ethanoic acid is the only weak acid of the four.

Advanced insight

Sulfuric acid releases its two hydrogens in stages. The first ionisation, giving H⁺ and HSO₄⁻, is essentially complete, but the second, HSO₄⁻ ⇌ H⁺ + SO₄²⁻, is only partial in moderately concentrated solutions. This is why salts such as sodium hydrogensulfate, NaHSO₄, exist and are themselves acidic.

Summary

The four common laboratory acids are hydrochloric (HCl), sulfuric (H₂SO₄), nitric (HNO₃) and ethanoic (CH₃COOH). All release H⁺ in water. The first three are strong and fully ionised; ethanoic acid is weak and only partly ionised. Each leaves a characteristic anion — Cl⁻, SO₄²⁻, NO₃⁻ or CH₃COO⁻ — that names the salts it forms.

Practice questions

1. Give the formula of hydrochloric acid and the name of its salts. Answer: HCl; its salts are chlorides. 2. Write the ionisation equation for sulfuric acid. Answer: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq). 3. Which of the four common acids is weak, and what symbol shows this in its equation? Answer: Ethanoic acid, CH₃COOH; its ionisation is shown with the reversible arrow ⇌. 4. Which acid would you react to make potassium nitrate? Answer: Nitric acid, HNO₃, because it supplies the nitrate ion.