Naming Compounds Practice: 40 Questions with Answers

Naming compounds and writing their formulas is a skill that becomes automatic only with plenty of practice. This set of 40 questions covers every common type: simple ionic compounds, polyatomic ions, transition metals with Roman numerals, covalent compounds with prefixes, acids and hydrates. The questions are grouped and graded. Work through a section, check your answers, then note any patterns in your mistakes before moving on.

Before you start: quick rules

1. Ionic (metal + non-metal, or with ammonium): name the cation, then the anion; balance charges for formulas (see how to write chemical formulas from names).
2. Transition metals: include the charge as a Roman numeral (see naming compounds with transition metals).
3. Covalent (two non-metals): use Greek prefixes; don’t balance charges (see how to name covalent compounds).
4. Acids: -ide → hydro-…-ic; -ate → -ic; -ite → -ous (see how to name acids).
5. Hydrates: salt name + Greek prefix + “hydrate” (see how to name hydrates).

A reference of ion charges is in common ions and their charges.

Section A: simple ionic compounds (1–8)

Name these:

1. NaCl
2. MgO
3. K₂S
4. CaBr₂

Write formulas for: 5. lithium oxide 6. aluminium chloride 7. barium fluoride 8. magnesium nitride

Section B: polyatomic ions (9–16)

Name these: 9. NaOH 10. CaCO₃ 11. (NH₄)₂SO₄ 12. KNO₃

Write formulas for: 13. sodium hydrogencarbonate 14. calcium phosphate 15. aluminium hydroxide 16. ammonium nitrate

Section C: transition metals (17–24)

Name these: 17. FeCl₂ 18. CuO 19. Fe₂(SO₄)₃ 20. PbO₂

Write formulas for: 21. copper(I) oxide 22. iron(III) hydroxide 23. chromium(III) chloride 24. tin(II) fluoride

Section D: covalent compounds (25–32)

Name these: 25. CO 26. N₂O₄ 27. PCl₃ 28. SF₆

Write formulas for: 29. carbon tetrachloride 30. dinitrogen pentoxide 31. sulfur dioxide 32. diphosphorus trioxide

Section E: acids and hydrates (33–40)

Name these: 33. HBr(aq) 34. H₂SO₄ 35. HNO₂ 36. CuSO₄·5H₂O

Write formulas for: 37. phosphoric acid 38. hypochlorous acid 39. magnesium sulfate heptahydrate 40. cobalt(II) chloride hexahydrate

Answer key

Section A

1. sodium chloride — Na⁺ and Cl⁻.
2. magnesium oxide — Mg²⁺ and O²⁻.
3. potassium sulfide — 2K⁺ and S²⁻ (note: sulfide, not sulfate).
4. calcium bromide.
5. Li₂O — 2 × (+1) balances −2.
6. AlCl₃ — +3 balanced by 3 × (−1).
7. BaF₂.
8. Mg₃N₂ — 3 × (+2) = +6; 2 × (−3) = −6.

Section B

9. sodium hydroxide.
10. calcium carbonate (chalk, limestone).
11. ammonium sulfate — a common fertiliser.
12. potassium nitrate.
13. NaHCO₃ — baking soda.
14. Ca₃(PO₄)₂ — 3 × (+2) = +6; 2 × (−3) = −6. Brackets are essential.
15. Al(OH)₃.
16. NH₄NO₃ — no brackets needed with one of each ion. Note it contains two nitrogen atoms in different ions.

Section C

17. iron(II) chloride — two Cl⁻ give −2, so Fe is +2.
18. copper(II) oxide — O²⁻ is −2, so Cu is +2.
19. iron(III) sulfate — three SO₄²⁻ give −6, shared by two Fe → +3 each.
20. lead(IV) oxide — two O²⁻ give −4 → Pb is +4.
21. Cu₂O — Cu⁺ and O²⁻.
22. Fe(OH)₃.
23. CrCl₃.
24. SnF₂ — the “stannous fluoride” in some toothpastes.

Section D

25. carbon monoxide — “mono” is used on the second element.
26. dinitrogen tetroxide (tetraoxide is also accepted). Don’t simplify to NO₂ — that’s a different compound, nitrogen dioxide.
27. phosphorus trichloride.
28. sulfur hexafluoride.
29. CCl₄ — “tetrachloromethane” is its systematic organic name.
30. N₂O₅.
31. SO₂.
32. P₂O₃ (the actual molecule is P₄O₆, but the name gives P₂O₃).

Section E

33. hydrobromic acid — bromide → hydro-…-ic.
34. sulfuric acid — sulfate → -ic.
35. nitrous acid — nitrite → -ous.
36. copper(II) sulfate pentahydrate — the blue crystals.
37. H₃PO₄ — phosphate is PO₄³⁻, so three H⁺.
38. HClO — hypochlorite is ClO⁻.
39. MgSO₄·7H₂O — Epsom salt.
40. CoCl₂·6H₂O — pink crystals.

Worked explanations for the trickiest questions

Question 3 (K₂S). Many students write “potassium sulfate”. The anion here is a single sulfur atom with no oxygen, so it’s the sulfide ion, S²⁻. Sulfate is SO₄²⁻, and potassium sulfate would be K₂SO₄. Whenever a simple anion has no oxygen, the name ends in -ide.

Question 14 (calcium phosphate). Calcium is Ca²⁺ and phosphate is PO₄³⁻. The lowest common multiple of 2 and 3 is 6, so three calcium ions (+6) balance two phosphate ions (−6). Because there are two phosphate ions, the phosphate goes in brackets: Ca₃(PO₄)₂. Writing Ca₃PO₄₂ would suggest a single strange ion with 42 oxygens.

Question 19 (Fe₂(SO₄)₃). Each sulfate carries −2, so three of them carry −6. That charge is shared by two iron ions, so each is +3, giving iron(III) sulfate. The subscript 2 on iron is not the charge.

Question 26 (N₂O₄). This is a covalent compound of two non-metals, so its name comes straight from the prefixes: dinitrogen tetroxide. Unlike ionic formulas, covalent formulas must never be simplified — NO₂ is a different, brown gas called nitrogen dioxide.

Question 38 (hypochlorous acid). Work backwards: “-ous acid” means the anion ends in “-ite”, so the anion is hypochlorite, ClO⁻. One negative charge needs one H⁺, giving HClO. It is the weak acid formed when chlorine dissolves in water, and it is what makes chlorinated water able to kill bacteria.

How did you do?

Score	What to do next
36–40	Excellent — try naming oxyanions and coordination compounds.
28–35	Good — review the section where you dropped marks.
20–27	Revisit the criss-cross method and the ion list, then retry sections B and C.
Below 20	Start with how to name ionic compounds and work through one section at a time.

Diagnosing your mistakes

Look at which errors you made — they usually fall into a few groups:

• Ionic vs covalent confusion. If you wrote “dinitrogen” for Mg₃N₂ or balanced charges for N₂O₄, practise deciding the compound type first: metal present → ionic; only non-metals → covalent.
• -ide / -ite / -ate mix-ups. K₂S is potassium sulfide. If this caught you, make flashcards for the common polyatomic ions.
• Missing brackets. Ca₃(PO₄)₂ and Al(OH)₃ need them whenever there’s more than one polyatomic ion.
• Roman numerals from atom counts. Fe₂(SO₄)₃ is iron(III), not iron(II) — calculate the charge from the anions.
• Simplifying covalent formulas. N₂O₄ must not be simplified.
• Acid endings. Remember: -ate → -ic, -ite → -ous, -ide → hydro-…-ic.

Tips for fluency

1. Learn a core set of polyatomic ions thoroughly: hydroxide, nitrate, sulfate, carbonate, phosphate, ammonium, hydrogencarbonate.
2. Always decide the type first.
3. Always check charges balance in ionic formulas.
4. Say names aloud — it helps fix the endings in memory.
5. Practise little and often: ten questions a day beats forty once a week.

You can check formulas and their molar masses with the molar mass calculator.

Key takeaways

• Identify the compound type before naming or writing a formula.
• Ionic: balance charges; bracket polyatomic ions; Roman numerals for variable-charge metals.
• Covalent: use prefixes; never simplify.
• Acids and hydrates have their own patterns built on the ion names.
• Track your error patterns and target them.