Which of the following represents the complete formation equation for solid cobalt(III) phosphate tetrahydrate, CoPO4·4H2O(s), from its elements in their standard states?

• A. Co(s) + P4(s) + 8O2(g) + 4H2O(l) → CoPO4·4H2O(s)
• B. Co3O4(s) + 2H3PO4(aq) + 4H2O(l) → 3CoPO4·4H2O(s)
• C. 2Co(s) + 2P(s) + 8O2(g) + 8H2(g) → 2CoPO4·4H2O(s)
• D. Co(s) + P(s) + 2O2(g) + 4H2O(l) → CoPO4·4H2O(s)

Answer: (D)

Verified step by step guidance

1. Identify the chemical formula of the compound: cobalt(III) phosphate tetrahydrate is given as CoPO4·4H2O(s). This means the compound contains cobalt (Co), phosphorus (P), oxygen (O), and water molecules (H2O) as part of its crystal structure.
2. Write the elements in their standard states as reactants. For cobalt, the standard state is solid Co(s). For phosphorus, the standard state is solid P(s) (usually white phosphorus as P4, but here we consider elemental P for the balanced equation). For oxygen, the standard state is O2(g). For hydrogen, since water is part of the hydrate, water itself (H2O(l)) is used rather than H2 gas.
3. Balance the equation so that the number of atoms of each element on the reactant side equals the number on the product side. The product is CoPO4·4H2O, so count atoms: 1 Co, 1 P, 4 O in phosphate, plus 4 H2O molecules (which add 8 H and 4 O).
4. Express the balanced formation reaction from elements in their standard states: Co(s) + P(s) + 2 O2(g) + 4 H2O(l) → CoPO4·4H2O(s). This shows one atom of cobalt, one atom of phosphorus, two molecules of oxygen gas (to provide 4 oxygen atoms), and four water molecules forming the hydrate.
5. Confirm that this equation represents the formation reaction, meaning it forms one mole of the compound from its elements in their standard states, which is the definition of a formation equation.