Copper sulphate 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 sulphate pentahydrate does not consist of 100 grams of copper sulphate. Chemists compensate for this by expressing the mass of copper sulphate as a percentage of the sample's total mass. In simplest terms, the concentration as per cent mass represents the number of grams of copper sulphate in a 100-gram sample of copper sulphate pentahydrate.

- Skill level:
- Moderately Challenging

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## Instructions

- 1
Calculate the formula weight of copper sulphate 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.

- 2
Calculate the formula weight of the copper sulphate, 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.

- 3
Determine the concentration by mass per cent by dividing the formula weight of CuSO4 by the formula weight of CuSO4-5H2O and multiplying by 100 per cent:

159.62 / 249.72 * 100 = 63.92 per cent.

This means that a 100-gram sample of copper sulphate pentahydrate will contain 63.92 grams of copper sulphate. It also means that copper sulphate pentahydrate contains 100 - 63.92 = 36.08 per cent water by mass.

#### Tips and warnings

- Many chemistry students conduct an experiment in which they determine the per cent water by mass in a sample of copper sulphate pentahydrate by heating the sample and measuring the mass lost during heating. See Resources for details.