Formulae for ionic compounds
3 marks · NormalCalculate the formula for the ionic compound formed between magnesium ions (Mg2+) and phosphate ions (PO4^3–). Show your working.
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Mg3(PO4)2
Common mistakes: incorrect use of brackets; failing to balance charges
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Marking points
M1 · 1 mark · Identifies each ion's charge: Mg2+, PO4^3–
M2 · 1 mark · Method used to balance charges (finds lowest common multiple of 2 and 3 is 6).
A1 · 1 mark · Correct answer: Mg3(PO4)2
Worked solution
Mg2+ and PO4^3–: To balance charges, find lowest common multiple of 2 and 3, which is 6. Need 3 Mg2+ (total +6) and 2 PO4^3– (total –6). So the formula is Mg3(PO4)2.