Copper sulfate pentahydrate, expressed in chemical notation as CuSO4-5H2O, represents a “hydrate.” Hydrates consist of an ionic substance -- a compound comprised of a metal and one or more nonmetals -- plus water molecules, where the water molecules actually integrate themselves into the solid structure of the ionic compound. This means, however, that a 100-gram sample of copper sulfate pentahydrate does not consist of 100 grams of copper sulfate. Chemists compensate for this by expressing the mass of copper sulfate as a percentage of the sample’s total mass. In simplest terms, the concentration as percent mass represents the number of grams of copper sulfate in a 100-gram sample of copper sulfate pentahydrate.
Calculate the formula weight of copper sulfate pentahydrate, CuSO4-5H2O. Multiply the atomic weight of each atom, as found on the periodic table of the elements, by the number of atoms in the formula and then add together all of the results. Copper, for example, exhibits an atomic mass of 63.55 atomic mass units, or amu, and the formula contains one Cu atom. Therefore, Cu = 63.55 * 1 = 63.55 amu. The other atoms in the formula are S = 32.07 * 1 = 32.07 amu; O = 16.00 * 9 = 144.00 amu; and H = 1.01 * 10 = 10.10 amu. Combine these figures to get the formula weight: 63.55 + 32.07 + 144.00 + 10.10 = 249.72 amu.
Calculate the formula weight of the copper sulfate, CuSO4, without the water. In this case, Cu = 63.55 * 1 = 63.55 amu; S = 32.07 * 1 = 32.07 amu; and O = 16.00 * 4 = 64.00 amu. These values total 159.62 amu.
Determine the concentration by mass percent by dividing the formula weight of CuSO4 by the formula weight of CuSO4-5H2O and multiplying by 100 percent:
159.62 / 249.72 * 100 = 63.92 percent.
This means that a 100-gram sample of copper sulfate pentahydrate will contain 63.92 grams of copper sulfate. It also means that copper sulfate pentahydrate contains 100 - 63.92 = 36.08 percent water by mass.
Many chemistry students conduct an experiment in which they determine the percent water by mass in a sample of copper sulfate pentahydrate by heating the sample and measuring the mass lost during heating. See Resources for details.