Introduction:
Many ionic compounds, when crystallized from a water solution, take up a definite proportion of water as a part of their structure. The compound containing this water is known as a hydrate and the water is known as the water of hydration. If the hydrate is strongly heated, this water of hydration can be “driven off” as a vapor. The solid remaining is known as the anhydrous salt.
Mass of water = Mass of hydrate – Mass of anhydrous salt or (Mass of water = Mass of compound before heating – Mass of compound after last heating)
The percentage of water in the hydrate can easily be calculated using the formula:
% Water = Mass of water x 100
Mass of hydrate
Formulas of Hydrates: Hydrates can contain varying amounts of water molecules in their structures. Some ionic compounds may have one, two, three, or more molecules of water associated with them. A common hydrate involves copper sulfate. There are five molecules of water associated with the compound copper sulfate. The formula for this hydrate is CuSO4 ·5H2O. The name given to CuSO4 ·5H2O is copper (II) sulfate pentahydrate.
Objective:
1. To determine the percentage of water in a hydrate and calculate percent error.
2. Honors only: Given the mass of one mole of the anhydrous salt, fine the empirical formula of the hydrate.
Materials:
Porcelain crucible, crucible tongs, ring stand, iron ring, pipe stem triangle, wire gauze, Bunsen burner set-up, and electronic balance.
Procedure:
1. Set-up the heating apparatus as demonstrated.
2. Heat the empty crucible for 2 minutes to drive off any moisture.
3. With the crucible tongs, carefully transfer the crucible to the wire gauze. Allow 1 minute to cool then mass the empty crucible.
4. Fill approximately 1/4 of the crucible with the hydrate. Mass and record.
5. Heat the crucible and its contents gently for 2 minutes and then more strongly for 3 minutes longer.
6. Transfer the crucible to the wire gauze and allow cooling for 1 minute.
7. Mass the crucible and its contents and record the mass of the 1st heating.
8. Transfer the crucible back to the heating apparatus and heat strongly for 3 minutes.
9. Transfer the crucible to the wire gauze and allow cooling for 1 minute.
10. Mass the crucible and its contents and record the mass of the 2nd heating.
11. If the difference in mass between 1st and 2nd heating is greater than 0.03 grams, repeat steps 9-11 to obtain a mass of a 3rd heating.
Data Table:
Constructed based on the procedure and allows enough space for appropriate answers. In this experiment, there should be two columns titling; sample and mass.
Analysis:
1. Determine the mass of the hydrate used.
2. Determine the mass of anhydrous salt.
3. Determine the mass of water.
4. Determine the percentage of water in the hydrate.
5. Obtain the actual percentage of water in this hydrate and determine your percent error.
6. Honors only: Obtain the mass of one mole of the anhydrous salt, to fine the empirical formula of the hydrate
Questions:
1. Can you suggest at least one reason why the above method might not be suitable for all hydrates?
2. Why is it important that the crucible be cooled to room temperature before massing?
3. What was the purpose of the second and possibly the third heating during the experiment?
4. Name the following hydrate: Na2CO3·4H2O
5. Write the formula of the following hydrate: calcium sulfate hexahydrate
Conclusion:
Remember to restate your objectives in the first paragraph and include your results when describing the procedure in the second paragraph. The last paragraph should include you opinion about the effectiveness of the lab procedure meeting the objectives that include sources of error.