Formulae for ionic compounds

2 marks · Hard

Iron(III) oxide is an ionic compound formed from Fe3+ and O2– ions. State the correct formula for iron(III) oxide and explain why this is the correct formula.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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Also accept: Fe2O3, with explanation such as: To balance charges, two Fe3+ ions (total +6) combine with three O2− ions (total −6), so the sum is zero.

Common mistakes: FeO3 or Fe3O2; Missing explanation

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Marking points

M1 · 1 mark · States or describes that the formula balances the charges: two Fe3+ ions and three O2– ions needed so charges balance.

A1 · 1 mark · Writes correct formula: Fe2O3.

Worked solution

Fe3+ has a 3+ charge, O2– has a 2– charge. The lowest total is when you have two Fe3+ ions (+6) and three O2– ions (–6). This balances charges.

The correct formula is Fe2O3.