Do I Need to Know About Hydrates?
Yes, but they’re nothing you can’t handle. A hydrate (or hydrated salt) is an
ionic substance in which water molecules bond to the ions in a fixed ratio. For
example, in copper sulfate pentahydrate, the ratio is given by the formula CuSO 4
• 5H 2 O. Anytime you see “• H 2 O” in a formula, you’re looking at a hydrate. You
might need to determine the percent composition of water in the hydrate (called
its water of hydration). If you do, you simply multiply the molecular weight of
water (18 amu) by the coefficient that precedes H 2 O in the formula. For
example, for a unit of CuSO 4 • 5H 2 O, the formula weight is approximately 64 +
32 + (4)(16) + 5(18) = 250 amu. The percentage of water in the hydrate is about
×100%, or 36%.