Copper(II) sulfate pentahydrate contains how many water molecules per formula unit?

• A. 3
• B. 4
• C. 5
• D. 6

Answer: (C)

Hydrated salts include water molecules tied into their crystal structure, called water of crystallization. The word pentahydrate signals five water molecules per formula unit. So for copper(II) sulfate pentahydrate, the full formula is CuSO4·5H2O, meaning there are five water molecules for each CuSO4 unit. Those five waters can be removed by heating, giving the anhydrous salt. If it were three, four, or six, that would correspond to different hydrates (trihydrate, tetrahydrate, or hexahydrate), not the pentahydrate shown.