Neutralisation of acids and salt production: use the formulae of common ions to deduce the formulae of salts.

The idea

A salt is an ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. To deduce a salt formula, split it into its cation and anion, write each ion's formula with its charge, then balance the total positive and negative charges so they cancel. For example, sodium ions Na⁺ and sulfate ions SO₄²⁻ need two Na⁺ for each SO₄²⁻, giving Na₂SO₄. Aluminium Al³⁺ with chloride Cl⁻ needs three Cl⁻, giving AlCl₃. Where a polyatomic ion is needed more than once, bracket it: calcium hydroxide is Ca(OH)₂. Charges are not shown in the final formula; they are only a working step. This skill underpins writing balanced equations for neutralisation and predicting products of acid reactions.

What earns the marks

• Identify the cation and anion present in the named salt, using the metal or ammonium ion as the cation and the acid-derived ion as the anion.
• State the charge on each common ion correctly, for example Na⁺, Mg²⁺, Al³⁺, Cl⁻, O²⁻, OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻.
• Apply the rule that total positive charge must equal total negative charge in the final neutral formula.
• Use the lowest whole-number ratio of ions, so magnesium chloride is MgCl₂ rather than Mg₂Cl₄.
• Place polyatomic ions in brackets when more than one is required, as in Ca(OH)₂ and Al₂(SO₄)₃.
• Write the final formula without charge symbols, since the compound itself is electrically neutral.

Common mistakes and how to put them right

• Writing the charges into the final formula, such as Na⁺Cl⁻ instead of NaCl.

  Put it right: Charges are a balancing step only; the finished salt formula shows no charge symbols.

• Swapping the ion ratio, for example writing MgCl instead of MgCl₂.

  Put it right: Check that 1 × 2⁺ balances 2 × 1⁻, so two chloride ions are needed.

• Forgetting brackets around a repeated polyatomic ion, writing CaOH₂ instead of Ca(OH)₂.

  Put it right: Bracket the whole ion before adding the subscript when more than one unit is needed.

• Using the acid's hydrogen count as the metal count without checking charge, for example assuming H₂SO₄ always gives a 2⁺ metal sulfate.

  Put it right: Balance charges from the ions themselves, not from the acid's formula.

Exam tips

• Write the two ion formulae with their charges side by side before combining them; this makes the balancing step visible and easy to check.
• Cross-check by multiplying the subscript by the ion charge for each ion and confirming the two totals are equal in size.
• If a question gives an unfamiliar salt name, break it into the familiar cation and anion parts rather than trying to recall the whole formula.
• Show the bracketed form for hydroxide, nitrate, sulfate and carbonate salts whenever the subscript is greater than one.