Experiment 2: The Percentage of Water in a Hydrate for AP Chemistry
For a quick review on stoichiometry, refer to the following concepts:
- Moles, Molar Mass, and Molarity for AP Chemistry
- Percent Composition and Empirical Formulas for AP Chemistry
- Reaction Stoichiometry for AP Chemistry
- Limiting Reactants for AP Chemistry
- Percent Yield for AP Chemistry
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.
- analytical balance
- crucible and cover
- support stand
- test tube(s)
- triangle crucible support
- wire gauze
- the mass of the crucible and cover
- the mass of the original sample and the crucible and cover
- 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.
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.
The experiment often uses hydrates of copper(II) sulfate, magnesium sulfate, calcium sulfate, or barium chloride.
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