Formulae for ionic compounds
5 marks · HardPotassium ions (K+) and phosphate ions (PO4^3–) combine to form an ionic compound.
State the symbol and charge of the potassium ion and the phosphate ion.
Explain, with working, how to deduce the simplest formula for the ionic compound formed from these ions.
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) — · must show K+, PO4^3–
(b) K3PO4, with explanation that three K+ ions are needed to balance the charge on one PO4^3– ion. · must show Balance: 3×(+1)=+3, 1×(–3)=–3; charges cancel
Common mistakes: KPO4 as the formula; K(PO4)3
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Marking points
B1 (a) · 1 mark · States symbol and charge of potassium ion: K+
B2 (a) · 1 mark · States symbol and charge of phosphate ion: PO4^3–
M1 (b) · 1 mark · Describes need to balance total charges (charges must cancel to zero for ionic compound).
M2 (b) · 1 mark · Shows working: 3 potassium ions are needed for one phosphate ion to balance charges: 3 × (+1) = +3 and 1 × (–3) = –3. (follow-through)
A1 (b) · 1 mark · Writes correct formula: K3PO4. (follow-through)
Worked solution
a) Potassium ion: K+, Phosphate ion: PO4^3–
b) The potassium ion is K+, so it has a charge of +1. The phosphate ion is PO4^3–, so –3. To balance the charges for a neutral compound, three K+ ions are needed for every PO4^3– ion.
Therefore, the formula is K3PO4.