AP Chemistry – Lab 02: Formula of a Hydrate
Many salts that have been crystallized from a water solution appear to be perfectly dry, yet when heated they yield large quantities of water. The crystals change form, even color sometimes, as the water is driven off. This suggests that water was present as part of the crystal structure. Such compounds are called hydrates. The ratio of moles of water present per moles of anhydrous (dry) salt is usually a whole number. One example is the hydrate copper sulfate. Its blue crystals look and feel dry. Yet, each mole of hydrate contains five moles of water. Its formula is CuSO4•5H2O. The dot between the CuSO4 and the 5H2O indicates that the water molecules are loosely attached to the other formula units. The molar mass CuSO4•5H2O is 249.71g
In this experiment, you will be given a hydrate whose formula is unknown. You will determine the mass of water driven off by heating and the amount of anhydrous salt that remains behind. At the end of the experiment, your teacher will give you the formula for the anhydrous salt of your unknown. You can then use this to determine its molar mass and use this information to determine the number of moles of anhydrous salt that are in your sample. Finally, you can determine the number of moles of water that are in your sample, and use this to determine theformula of the hydrate.
- Place a dry, clean crucible with a lid in a clay triangle mounted on an iron ring. Leave the cover askew so that any water in the crucible can easily escape. Heat with a Bunsen burner for 2-3 minutes to clean and dry the crucible.
- Do not touch the clean crucible. Do not weigh the crucible while it is still hot! After you have allowed it to cool for a few minutes, use crucible tongs to transfer the crucible and the lid to your balance. Record the mass of the empty crucible and lid.
- Add hydrate crystals to the crucible until it is 1/4-1/3 full (about 1 to 5 g). Weigh the crucible and its contents with the lid. Record the mass of the crucible, lid, and unknown hydrate. Record the number or letter of your unknown in your lab manual.
- Place the crucible, with its lid only slightly askew, on the triangle and heat it very gently so as to avoid spattering. Gradually increase the flame until the crucible bottom is at most a dull red. Adjust the cover until there is a 0.5 cm opening between it and theopening. (check teacher demo). Maintain this temperature for five minutes. Turn off the burner and allow the crucible to cool for at least five minutes with the cover closed completely.
- When the crucible has cooled, weigh the crucible, lid, and the contents.
- In order to ensure that all the water was driven off from the hydrate, reheat the crucible, its contents, and its lid. After allowing it to cool again (with the lid completely on), reweigh the crucible and its contents. If your results come within 0.03 g of the mass of the previous heating, begin your calculations. If your results are not within 0.03 grams of your previous heating, reheat, cool, and weigh until it does! (This may take several heatings).