Formulae for ionic compounds
3 marks · EasyState the formula of the calcium ion and the oxide ion.
What is the simplest formula for the ionic compound formed from these ions?
Explain why the compound has this 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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(a) Ca2+, O2−
(b) CaO
(c) The charges on Ca2+ and O2− cancel each other so only one of each ion is needed for an electrically neutral compound.
Common mistakes: Getting ion charges wrong (e.g., Ca+ or O−2); Incorrect overall formula (e.g., CaO2); Confusing oxide with peroxide
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Marking points
B1 (a) · 1 mark · Correct formulae for Ca2+ and O2− ions (both needed for the mark).
B1 (b) · 1 mark · Correct formula CaO.
B1 (c) · 1 mark · Clear explanation that charges balance/cancel, so one of each ion; recognises need for neutrality.
Worked solution
(a) The calcium ion is Ca2+. The oxide ion is O2−.
(b) The formula for the compound is CaO.
(c) Ca2+ has a 2+ charge; O2− has a 2− charge. When combined in a 1:1 ratio, the charges cancel out to give a neutral compound, so the formula is CaO.