How to Find Percent Composition of Hydrate
Hydrates are typically inorganic salts that contain water in the form of bound H2O molecules. An example of hydrates is copper sulfate pentahydrate (CuSO4-5H2O). Hydrates are relatively stable at room temperature because the water is integrated into the salt crystal lattice, but usually readily lose water upon heating. Calculating the percent composition of hydrates is a frequent assignment in chemistry courses.
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Instructions
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1
Write down the chemical formula of the hydrate. For example, the Glauber's salt (sodium sulfate decahydrate) has the formula Na2SO4-10H2O.
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2
Obtain atomic masses of all elements that compose the hydrate using the Periodic Table of Elements. In this example, the hydrate is composed of sodium (Na), sulfur (S), oxygen (O) and hydrogen (H). The atomic masses of the elements are 23, 32, 16 and 1, respectively.
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3
Multiply the atomic mass of the element by the number of the respective atoms in the salt part of the hydrate to calculate the mass contributions for each element. In this example, the salt is Na2SO4 and the mass contributions of the elements are 23 x 2 = 46 (sodium), 32 x 1 (sulfur) and 16 x 4 = 64 (oxygen).
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4
Compute the molecular weight of water (H2O) that is 16 x 1 + 1 x 2 = 18.
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5
Multiply the molecular weight of water by the number of water molecules to calculate the mass of the water in the hydrate. In the example, this mass is 18 x 10 = 180.
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Add up the mass of the water and the mass contributions of all other elements to calculate the total molecular weight of the hydrate. In this example, the molecular weight of the hydrate is 180 + 46 + 32 + 64 = 322.
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Divide the mass of water molecules by the molecular weight of the hydrate, and then multiply the result by 100 to calculate the percent of water. In this example, the percentage of water is (180 / 322) x 100 = 55.9 percent.
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Divide the mass contribution of sodium by the molecular weight of the hydrate, and then multiply the result by 100 to calculate the percent of sodium. In this example, the percentage of sodium is (46 / 322) x 100 = 14.4 percent.
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9
Divide the mass contribution of sulfur by the molecular weight of the hydrate, and then multiply the result by 100 to calculate the percent of sulfur. In the example, the percentage of sulfur is (32 / 322) x 100 = 9.9 percent.
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10
Divide the mass contribution of oxygen by the molecular weight of the hydrate, and then multiply the result by 100 to calculate the percent of oxygen. In this example, the percentage of oxygen is (64 / 322) x 100 = 19.9 percent.
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