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Experiment 2: The Percentage of Water in a Hydrate for AP Chemistry

By — McGraw-Hill Professional
Updated on Feb 2, 2011

For a quick review on stoichiometry, refer to the following concepts:

Synopsis

The amount of water present in a sample is determined by weighing a compound before and after heating. The difference in mass is due to the loss of water.

Equipment

      analytical balance
      burner
      crucible and cover
      support stand
      test tube(s)
      tongs
      triangle crucible support
      wire gauze

Measurements

  1. the mass of the crucible and cover
  2. the mass of the original sample and the crucible and cover
  3. the mass of the heated (dried) sample and the crucible and cover

The last measurement must be done after the sample has cooled to room temperature.

Calculations

The mass of the hydrate is calculated from the difference between masses 1 and 2. The mass of the water lost is calculated from the difference between masses 2 and 3. The percentage of water is calculated by dividing the mass of the water lost by the mass of the hydrate and multiplying the result by 100%.

A variation of this experiment uses the mass of the anhydrous material (calculated from the difference between masses 1 and 3). The moles of the anhydrous material and water are then calculated from their respective masses and molar masses. The simplest ratio of the moles gives the empirical formula.

Comments

The experiment often uses hydrates of copper(II) sulfate, magnesium sulfate, calcium sulfate, or barium chloride.

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