Zinc phosphate is an ionic compound made from zinc cations and phosphate anions. The correct chemical formula is Zn3(PO4)2.
Ions present and charge neutrality
Zinc commonly forms Zn2+, and the phosphate ion is PO43−. Charge neutrality in an ionic compound requires the total positive charge to equal the total negative charge.
The smallest whole-number combination that balances charges uses three zinc ions and two phosphate ions: \[ 3(\,+2\,) + 2(\,-3\,) = 0. \]
| Ion | Charge | Count in formula unit | Total charge contribution |
|---|---|---|---|
| Zn2+ | \(+2\) | 3 | \(+6\) |
| PO43− | \(-3\) | 2 | \(-6\) |
| Net | 0 |
Formula and name
The balanced ion ratio gives Zn3(PO4)2. Parentheses are used because phosphate is a polyatomic ion; the subscript 2 applies to the entire PO4 group.
The name zinc phosphate is standard because zinc is overwhelmingly found as Zn2+ in general chemistry contexts. The name zinc(II) phosphate is also acceptable and explicitly states the oxidation state.
Atoms in one formula unit
One formula unit of Zn3(PO4)2 contains 3 Zn atoms, 2 P atoms, and 8 O atoms.
Molar mass (useful in stoichiometry)
Using common atomic masses \(M(\mathrm{Zn})=65.38\), \(M(\mathrm{P})=30.97\), \(M(\mathrm{O})=16.00\) (g/mol), the molar mass is: \[ M\big(\mathrm{Zn_3(PO_4)_2}\big)=3(65.38)+2(30.97)+8(16.00)=196.14+61.94+128.00=386.08\ \text{g/mol}. \]
Aqueous behavior and precipitation
Zinc phosphate is sparingly soluble in water and is often encountered as a precipitate in reactions that mix soluble zinc salts with soluble phosphate salts. The dissolution equilibrium can be written as: \[ \mathrm{Zn_3(PO_4)_2(s) \rightleftharpoons 3\,Zn^{2+}(aq) + 2\,PO_4^{3-}(aq)}. \] The solubility product expression is: \[ K_{sp} = [\mathrm{Zn^{2+}}]^3[\mathrm{PO_4^{3-}}]^2. \]
Visualization: ion ratio that produces Zn3(PO4)2
Common pitfalls
- Incorrect subscripts: ZnPO4 is not charge-balanced for Zn2+ and PO43−.
- Missing parentheses: Zn3PO42 is ambiguous; Zn3(PO4)2 clearly indicates two phosphate groups.
- Confusing phosphate with other oxyanions: Phosphate is PO43−; changing the oxygen count or charge changes the compound.