Naming Acids

Key Ideas

• Acid: releases H⁺ in water (Arrhenius definition); formulas usually written with H first.
• Binary acid: H + one nonmetal, with no oxygen. Named hydro- + nonmetal stem + -ic + acid.
  • HCl → hydrochloric acid; HBr → hydrobromic acid; H₂S → hydrosulfuric acid; HF → hydrofluoric acid.
• Oxyacid: H + oxygen + a third element. Named from the polyatomic ion:
  • ion ends in -ate → acid ends in -ic acid (NO₃⁻ nitrate → HNO₃ nitric acid; SO₄²⁻ sulfate → H₂SO₄ sulfuric acid).
  • ion ends in -ite → acid ends in -ous acid (NO₂⁻ nitrite → HNO₂ nitrous acid; SO₃²⁻ sulfite → H₂SO₃ sulfurous acid).
• per-…-ate ion → per-…-ic acid (ClO₄⁻ perchlorate → HClO₄ perchloric acid).
• hypo-…-ite ion → hypo-…-ous acid (ClO⁻ hypochlorite → HClO hypochlorous acid).
• The "-ic acid"/"-ous acid" distinction follows a memory aid: "-ate → -ic" (both rhyme-ish), and "-ite → -ous" (the other one).

Equations and Variables

This topic is qualitative, but one ion-accounting rule matters: an acid's hydrogen count equals the charge of the polyatomic ion. For example, SO₄²⁻ has charge −2, so two H⁺ are needed → H₂SO₄; PO₄³⁻ has charge −3 → H₃PO₄. Nitrate NO₃⁻ (charge −1) → HNO₃.

Reference family (chlorine oxyacids), listed from most to fewest oxygens:

• HClO₄ perchloric acid (perchlorate, ClO₄⁻)
• HClO₃ chloric acid (chlorate, ClO₃⁻)
• HClO₂ chlorous acid (chlorite, ClO₂⁻)
• HClO hypochlorous acid (hypochlorite, ClO⁻)

How It Works or Problem-Solving Method

Naming an acid from its formula:

1. Does it contain oxygen?
   • No → binary acid: "hydro-" + stem + "-ic acid."
   • Yes → oxyacid: continue.
2. Identify the polyatomic ion (drop the H⁺ ions; the remaining group with its charge is the ion).
3. Match the ion's suffix/prefix to the acid name:
   • -ate → -ic acid; per-…-ate → per-…-ic acid
   • -ite → -ous acid; hypo-…-ite → hypo-…-ous acid.

Writing a formula from an acid name:

1. Reverse the suffix mapping to recover the ion (and its charge).
2. Add enough H⁺ to balance the ion's charge.
3. For binary acids, recall the nonmetal from the "hydro-…-ic" stem.

Worked Example

Problem 1: Name HCl, H₂S, and HF.

All are H + one nonmetal with no oxygen → binary acids: HCl → hydrochloric acid; H₂S → hydrosulfuric acid; HF → hydrofluoric acid.

Problem 2: Name HNO₃ and HNO₂.

HNO₃ comes from nitrate (NO₃⁻, -ate) → nitric acid. HNO₂ comes from nitrite (NO₂⁻, -ite) → nitrous acid.

Problem 3: Name H₂SO₄ and H₂SO₃.

H₂SO₄ from sulfate (SO₄²⁻) → sulfuric acid. H₂SO₃ from sulfite (SO₃²⁻) → sulfurous acid.

Problem 4: Name HClO₄ and write the formula for hypochlorous acid.

HClO₄ from perchlorate (ClO₄⁻) → perchloric acid. Hypochlorous acid ← hypochlorite (ClO⁻), charge −1 → one H⁺ → HClO.

Problem 5: Write the formula for phosphoric acid.

Phosphoric acid ← phosphate (PO₄³⁻), charge −3 → three H⁺ → H₃PO₄.

Common Confusions

• "Strong acid and concentrated acid mean the same thing." Wrong — strong means the acid ionizes completely in water (HCl, HNO₃, H₂SO₄), a property of the substance's identity; concentrated means a lot of acid is dissolved in a little water. You can have dilute strong acid and concentrated weak acid.
• "HCl is 'chloric acid.'" Wrong — HCl is a binary acid: hydrochloric acid. "Chloric acid" (HClO₃) is an oxyacid containing oxygen.
• "Hydrochloric acid and chloric acid are the same." Wrong — HCl (no oxygen) vs. HClO₃ (with oxygen) are different compounds with different formulas and properties.
• "Sulfuric and sulfurous acids differ only in spelling." Wrong — sulfuric acid is H₂SO₄ (from sulfate) and sulfurous acid is H₂SO₃ (from sulfite); one fewer oxygen atom changes both the formula and the chemistry.