Phosphate, PO43−, is a 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 lithium: Li3PO4
Combined with iron(III): Fe3(PO4)3
Combined with iron(III): FePO4
Combined with copper(I): Cu3PO4
Combined with copper(I): (Cu)3PO4
Combined with magnesium: Mg3(PO4)2
Combined with magnesium: Mg2(PO4)3
Combined with lithium: Li2PO4
The Correct Answer and Explanation is:
To determine which formulas correctly represent phosphate compounds, we must follow these steps:
1. Understand the phosphate ion charge
Phosphate ((PO_4^{3-})) is a polyatomic ion with a charge of -3. To form a neutral compound, the total positive charge of the cations must balance the -3 charge of phosphate.
2. Analyze each formula
- Combined with lithium ((Li^+)):
Lithium has a charge of +1. To balance the -3 charge of (PO_4^{3-}), three (Li^+) ions are required. Correct formula: (Li_3PO_4).
The formula (Li_2PO_4) is incorrect because it does not balance the charge. - Combined with iron(III) ((Fe^{3+})):
Iron(III) has a charge of +3. One (Fe^{3+}) ion can balance one (PO_4^{3-}). Correct formula: (FePO_4).
The formula (Fe_3(PO_4)_3) is mathematically correct (3:3 ratio), but it simplifies to (FePO_4), so it is redundant but valid. - Combined with copper(I) ((Cu^+)):
Copper(I) has a charge of +1. Three (Cu^+) ions are needed to balance the -3 charge of phosphate. Correct formula: (Cu_3PO_4).
The formula ((Cu)_3PO_4) is incorrect because parentheses are unnecessary for a single element. - Combined with magnesium ((Mg^{2+})):
Magnesium has a charge of +2. To balance (PO_4^{3-}), a ratio of 3 (Mg^{2+}) to 2 (PO_4^{3-}) is needed. Correct formula: (Mg_3(PO_4)_2).
The formula (Mg_2(PO_4)_3) is incorrect because the charges do not balance.
Correct Formulas
- (Li_3PO_4)
- (FePO_4)
- (Cu_3PO_4)
- (Mg_3(PO_4)_2)
Explanation
Formulas of ionic compounds are based on charge neutrality. The charges of the cations and anions must cancel out to produce a compound with no net charge. Using this principle, we balance the number of ions needed for each compound.