How to memorize the strong acids and bases
The prettysmart teamUpdated 3 min readthe short answer
Learn 6 strong acids (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄) and the group 1 hydroxides plus Ca, Sr and Ba; every other acid and base you meet is weak. It takes about 4 short days: the halogens and the three oxyacids first, then the bases, then the traps.
Key takeaways
- "So I Brought No Clean Clothes" gives the 6 acids: H₂SO₄, HI, HBr, HNO₃, HCl, HClO₄.
- HF is the trap: the one hydrogen halide that is a weak acid.
- A group 2 hydroxide gives two OH⁻, so 0.010 M Ba(OH)₂ has 0.020 M OH⁻.
In this guide
The strong acids and bases list
| Strong acids | Name | Strong bases | Name |
|---|---|---|---|
| HCl | hydrochloric acid | LiOH | lithium hydroxide |
| HBr | hydrobromic acid | NaOH | sodium hydroxide |
| HI | hydroiodic acid | KOH | potassium hydroxide |
| HNO₃ | nitric acid | RbOH, CsOH | rubidium, cesium hydroxide |
| H₂SO₄ | sulfuric acid | Ca(OH)₂, Sr(OH)₂ | calcium, strontium hydroxide |
| HClO₄ | perchloric acid | Ba(OH)₂ | barium hydroxide |
“Strong” means it ionizes completely in water. Treat anything not on this table as weak unless your class’s list adds it.
The trick: two rows of acids, two groups of bases
The list is a periodic-table pattern, so you learn 4 rows instead of 14 formulas:
- Halogen acids: hydrogen plus a halogen, from chlorine down. Acidity rises down the group, HF < HCl < HBr < HI, and HF is the one that’s weak.
- Oxyacids: the top-oxidation-state acid of nitrogen, sulfur and chlorine: HNO₃, H₂SO₄, HClO₄. A higher oxidation number on the central atom makes an oxyacid stronger, which is why HNO₂ and H₂SO₃ lose out.
- Bases: every group 1 hydroxide, plus calcium, strontium and barium from group 2.
Mnemonic: So I Brought No Clean Clothes: sulfuric, HI, HBr, nitric, HCl, perchloric.
The traps that cost points
| Looks strong | Why it isn’t |
|---|---|
| HF | Weak (Kₐ = 6.8 × 10⁻⁴), though HCl, HBr and HI are strong |
| HClO₃, chloric acid | On some lists, not on OpenStax’s or AP’s; perchloric, HClO₄, is on all of them |
| HSO₄⁻ | Weak (Kₐ = 1.2 × 10⁻²): what’s left after H₂SO₄ loses its first H⁺ |
| Acetic acid, CH₃COOH | Weak (Kₐ = 1.8 × 10⁻⁵), like other carboxylic acids |
| NH₃ | A weak base (Kb = 1.8 × 10⁻⁵): no OH⁻ in its formula |
What knowing them does for you
A strong acid’s concentration is its [H₃O⁺], so 0.0010 M HNO₃ has pH 3.00 with no equilibrium table (AP 8.2.A.1). A group 2 hydroxide counts double: 0.010 M Ca(OH)₂ gives 0.020 M OH⁻, pOH 1.70, pH 12.30 (8.2.A.2). A weak acid needs Kₐ and an ICE table, which is why spotting strong ones first saves time on every pH question.
A 4-day plan
Spread it over days: the same practice split across days sticks better than one long session (Cepeda et al., 2006).
| Day | What to do |
|---|---|
| 1 | The 6 acids with the mnemonic. Write formulas and names from memory 3 times |
| 2 | The bases by group. Write the full table from a blank page |
| 3 | The traps table, then sort 20 formulas into strong and weak |
| 4 | The full table once more, then 5 pH problems with strong acids and bases |
Test tomorrow? Learn the 4 rows tonight, write the table twice, sleep, and write it once more before class.
Test yourself instead of rereading: 8 of the deck’s 42 cards are below, and the whole deck goes into prettysmart with one code.
Same chapter, other lists: polyatomic ions and their charges name the oxyacids’ anions, and the solubility rules say which hydroxides dissolve.
Flashcards: Strong acids and strong bases
8 of the 42 cards, each checked against this guide's sources. Tap a card to see the answer.
1Formula of perchloric acid?
2HF: strong or weak acid?
3Is HClO₃ (chloric acid) on the AP Chemistry list of strong acids?
4HSO₄⁻, left after H₂SO₄ loses one H⁺: strong or weak acid?
5Which group 2 hydroxides are on OpenStax's list of strong bases?
6NH₃ (ammonia): strong or weak base?
7[OH⁻] in 0.010 M Ba(OH)₂?
8pOH of 0.010 M Ca(OH)₂?
Questions people ask
Are there 6 or 7 strong acids?
OpenStax and the AP Chemistry course description both name 6. Some lists add chloric acid, HClO₃, as a seventh, and a few add more oxyacids. Learn the 6, then check your class's list.
Is Mg(OH)₂ a strong base?
It isn't on OpenStax's list, which stops at Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂. AP's course description says "group I and II hydroxides" as an example, so follow your class's list.
Sources
- Chemistry 2e, 14.3 Relative Strengths of Acids and Bases, OpenStax
- AP Chemistry Course and Exam Description (effective fall 2024), topics 8.2 to 8.6, College Board
- MCAT Mnemonics: Strong Acids, MedSchoolCoach
- Distributed Practice in Verbal Recall Tasks (Cepeda et al., 2006), Psychological Bulletin