Chapter 5 · Molecules and Compounds
5.9Naming Acids
10 min · two checks
Predict
HNO₃ is nitric acid. What is HNO₂?
The idea
- Name binary acids and oxyacids.
- Connect an acid name to its anion.
Binary acids contain hydrogen and one other nonmetal, with no oxygen. In water they are named hydro- plus the anion root plus -ic acid. HCl is hydrochloric acid, H₂S is hydrosulfuric acid, HBr is hydrobromic acid. When the same compounds are gases, not dissolved in water, they can be named as molecular compounds: hydrogen chloride. Context matters.
Oxyacids contain hydrogen, oxygen, and another element. They do not use hydro-. If the anion ends in -ate, the acid ends in -ic: NO₃⁻ nitrate → HNO₃ nitric acid; SO₄²⁻ sulfate → H₂SO₄ sulfuric acid; PO₄³⁻ → H₃PO₄ phosphoric acid. If the anion ends in -ite, the acid ends in -ous: NO₂⁻ nitrite → HNO₂ nitrous acid; SO₃²⁻ → H₂SO₃ sulfurous acid. The number of hydrogens equals the charge magnitude of the anion, so the formula is neutral.
Keep these
- Binary acid: hydro- … -ic acid.
- -ate anion → -ic acid. -ite anion → -ous acid.
- Hydrogens in the formula balance the anion charge.
Worked path
Name HCl (as an acid) and write the formula of phosphoric acid.
Observe
HCl has no oxygen. Phosphoric acid comes from phosphate, PO₄³⁻.