How to Calculate Moles of Hydration

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Some chemicals tend to absorb moisture out of the atmosphere. In some cases, the water molecules actually incorporate directly into the chemical structure of a solid. Chemists refer to this water as the "water of hydration" and they represent it with chemical formulas such as MgSO4.7H2O, known as magnesium sulfate heptahydrate, or Epsom salts. In this case, the 7H2O means that the compound absorbs seven water molecules per formula unit. Alternatively, a chemist would say that it absorbs seven moles of water per mole of magnesium sulfate. Experimentally, chemists determine the moles of hydration by weighing a sample of the material before and after heating it to dryness. The difference in weight represented the amount of water driven off, which allows the chemist to calculate the moles of water driven off.

Things You'll Need

  • Water of hydration data, including mass of the sample before and after heating
  • Calculator
  • Calculate the amount of water in the sample by subtracting the weight of the sample before and after it was heated. For example, a 1.045-gram sample of hydrated copper(II) sulfate, CuSO4.xH2O, that weighs 0.662 grams after it was heated lost 1.045 - 0.662 = 0.383 grams of water.

  • Determine the moles of water driven out of the sample by dividing the mass of water lost by water's molecular weight, which is 18.02. Continuing the previous example, 0.383 / 18.02 = 0.0213 moles of water.

  • Calculate the moles of the compound remaining after the water was driven off. This is determined by dividing the mass of the residue in grams by the compound's formula weight, as determined by adding together the atomic weights of all of the elements in its chemical formula. In the case of CuSO4.xH2O, the residue would contain everything except the water, or CuSO4. This formula contains one copper, one sulfur and four oxygen atoms, which possess atomic weights of 63.55, 32.07 and 16.00, respectively, as found on the periodic table of the elements. The formula weight of CuSO4 is therefore 159.62. The 0.662 grams of CuSO4; therefore, represents 0.662 / 159.62 = 0.00415 moles of CuSO4.

  • Find the moles of hydration by dividing the moles of water by the moles of residue. In this case, the hydrated compound contained 0.0213 moles of water and 0.00415 moles of CuSO4. The moles of hydration is therefore 0.0213 / 0.00415 = 5.13.

  • Round the moles of hydration to the nearest integer and write the hydrated formula of the compound. Continuing the previous example, 5.13 round to 5; therefore, the formula of the hydrated salt is CuSO4.5H2O (ref 4).

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