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

• A. 2
• B. 5
• C. 1
• D. 0

Answer: (B)

Water of crystallization is the water molecules embedded in the solid salt's crystal lattice. When a salt is described as a pentahydrate, it means there are five such water molecules for each formula unit of the salt. Copper sulfate pentahydrate has the formula CuSO4·5H2O, so there are five water molecules per CuSO4 unit. These waters are part of the crystal structure rather than chemically bonded to copper in the salt, and they can be lost on heating, which is why heating the blue pentahydrate yields white anhydrous CuSO4. The numbers 2, 1, or 0 would correspond to dihydrate, monohydrate, or anhydrous forms, respectively.