Phosphate; PO4 polyatomic ion that can combine with any cation to form an ionic compound. Identify all the formulas below that correctly represent phosphate compounds. Select one or more: Combined with magnesium: Mg3(PO4)2 Combined with iron(III): Fe3(PO4)2 Combined with iron(II): FePO4 Combined with lithium: Li3PO4 Combined with copper(V): Cu3(PO4)2 Combined with copper(II): CuPO4 Combined with magnesium: Mg3(PO4)2
The Correct Answer and Explanation is:
The correct formulas that represent phosphate compounds from the given options are:
- Fe₃(PO₄)₃
- Li₃PO₄
- Cu₃PO₄
- Mg₃(PO₄)₂
Here’s the explanation:
Phosphate is a polyatomic ion with the formula PO₄³⁻. In ionic compounds, it combines with positively charged cations. To form a neutral compound, the total positive and negative charges must balance.
Start with magnesium. Magnesium forms Mg²⁺ ions. To balance with PO₄³⁻, we need 3 Mg²⁺ ions for every 2 PO₄³⁻ ions. This makes the formula Mg₃(PO₄)₂, which is correct. The option Mg₂(PO₄)₃ is incorrect since it would not balance the charges.
For iron(III), the ion is Fe³⁺. To balance with PO₄³⁻, which also has a 3- charge, a 1:1 ratio works. So FePO₄ would seem correct by charge, but it actually misrepresents iron’s oxidation state. FePO₄ is the formula for iron(III) phosphate only if iron is trivalent, which it is in this case. But Fe₃(PO₄)₃ shows the correct stoichiometry and is definitively correct.
For lithium, the ion is Li⁺. Three lithium ions are needed to balance one phosphate ion, giving Li₃PO₄. This is correct and appears twice in the options.
For copper(I), the ion is Cu⁺. Just like lithium, three Cu⁺ ions are needed per PO₄³⁻ ion. So Cu₃PO₄ is correct. The other copper option (Cu)₃PO₄ is also correct in formula but redundant notation.
In summary, the correct formulas are those where the total ionic charges balance out to form a neutral compound. That is the key principle guiding all ionic compound formation.