COMPOSITION AND FORMULA OF A HYDRATE
Purpose
The purpose of this lab is to determine the formula of a hydrate.
Background
Hydrates are crystalline compounds in which a fixed number of water molecules are combined with each formula unit of the salt. The formula of a hydrate shows the formula of the anhydrous salt, followed by a dot and the number of water molecules per unit of anhydride:
CoCl2 • 6 H2O CoCl2 + 6 H2O
cobalt (II) chlorideanhydrous
hexahydrate cobalt (II) chloride
Often the water is not tightly bound and can be driven off by heating without decomposing the salt, so if you weigh the sample before and after heating, you can determine the mass of water included in the crystal. The purpose of this experiment is to determine percent water and the formula of a hydrate.
This experiment is an example of gravimetric analysis, which is an analysis that depends entirely on accurate weighing of dry samples. In gravimetric analysis, great care must be taken that the sample and container are room temperature, dry, and free of contamination. You will be using a porcelain crucible to heat and weigh your sample. The crucible must be handled with crucible tongs at all times. Leave the crucible on its clay support to cool completely before attempting to weigh it: hot objects do not give correct masses, and if you put a hot crucible on a tabletop or mat, it often breaks. Patience is a virtue in gravimetric analysis (bringing along something to do while you wait helps, too).
Procedure
1. Obtain a crucible and cup of sample. Support a clay triangle on a ring above the bunsen burner, and adjust the ring height so that the bright inner cone of the flame will be just below the bottom of the crucible. Put the empty crucible on the triangle, put on the lid (slightly ajar), and heat it in the hottest part of the flame until the bottom glows red-hot for 5 minutes. Turn off the flame and allow the crucible to cool to room temperature (this usually takes 10–15 minutes).
2. Record the mass of the empty crucible + lid, then add the sample to the crucible and weigh it (with lid). Put the crucible on its triangle and put the lid on, again just slightly ajar (you want to let water vapor escape, but you don’t want sample to spatter out). Heat the crucible gently at first, then gradually increase the intensity of the heat until the crucible is again being heated in the hottest part of the flame and glows red for at least 5 minutes. Allow it to cool and weigh it.
3. Put the crucible back on the triangle and repeat the heating, gently and then strongly for at least 5 minutes. Cool and weigh. The two masses should agree within a few milligrams; if they don’t, repeat the heating and cooling until they do (this is called heating to constant mass).
4. When you are finished, put the anhydrous salt into the waste container and rinse out the crucible thoroughly with deionized water. Leave it in the instructor’s tray to dry.
Analysis
1. Calculate the percent water in the hydrate, with attention to sigfigs.
2. The salt you used was CuSO4 • x H2O, and the anhydride is CuSO4. Convert the mass anhydride to moles CuSO4, and mass water in the sample to moles H2O. Calculate the mole ratio of water to anhydride, which gives you the value of x in the formula. Round off appropriately (to an integer or a half, such as ), and write the formula of the hydrate.
Discussion
Obtain the accepted formula for the hydrate, compare it to your result, and discuss your accuracy.
Answer these questions about possible problems with this procedure:
1. Why is it important to heat to constant mass?
2. How would the result be affected if some sample spattered out of the crucible during heating?
3. How might the result be affected if the salt itself underwent a chemical change during heating?