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Chemistry Lab: Composition of a Hydrate

This is the first time in about 7 years that I have taught a chemistry class.  My usual teaching assignment is a full day of AP Biology. My school made the transition this year from a 5 period day to a 7 period day, thus the need to assign teachers additional classes to teach.  So this year I am doing chemistry labs that I have not done for some time..... and I am being reminded how much I love them! I love the mathematical and analytical nature of chemistry labs and the need for exact and precise laboratory procedures.

Lately, I have been teaching how to write chemical formulas and the naming of compounds.  So this past week we did a lab called "Composition of Hydrates."  Hydrates are compounds that have some number of water molecules attached to them.  The premise of the lab is simple:  A known mass of a hydrate is heated to release the water of hydration.  The mass of the resulting anhydrous salt is determined.  The difference in the mass of the hydrate and the mass of the anhydrous salt can be used to determine the formula of the compound.

In our lab, we used copper (II) sulfate pentahydrate. A fine crystal is needed for the lab.  Since we only had the medium crystal in stock, my students used a mortar and pestle to grind the crystals into a finer powder.

The hydrate is heated over a Bunsen burner in an evaporating dish to remove the water of hydration.  But first the empty evaporating dish must be heated to remove all water, allowed to cool briefly, and the mass of the empty dish determined.

Heating the hydrate to remove the water of hydration.

The anhydrous salt.
The difference in the "before mass" and the "after mass" is used to determine the mass of water lost.  Students are given directions in how to use this information to determine that the compound used was copper (II) sulfate pentahydrate.  I love the simplicity of this lab and the fact that my students clearly understood how to use this data to determine the formula of the compound.  All in all, a great day!

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