A number of ionic compounds contain one or more waters of hydration in their formulas.  A good example of this is copper(II) sulfate which exists in an anhydrous form, CuSO4(s), as well as a pentahydrate form, CuSO4•5H2O.

Many anhydrous compounds have a strong tendency to absorb water vapor from the air, thus becoming hydrated compounds.  These anhydrous compounds find use as moisture reducing agents.  You may have noticed that containers of such compounds are often found in bottles containing pills that would decompose if moisture were present.  Such compounds are said to be hygroscopic.  Some of these compounds absorb water to such an extent that they actually dissolve in the water that they take up.  When this is the case, the compounds are said to be deliquescent.  Sodium hydroxide is an example of this type of compound.  On the other hand, some hydrated compounds tend to spontaneously loose their water of hydration when they are placed in a dry environment.  These compounds are said to be efflorescent.

In this experiment, the number of water molecules associated with each formula unit of a salt will be determined.  The amount of water in the hydrated compound will be determined by heating a massed sample of the compound in order to drive off the water.  From the mass change, the percent water in the sample will be determined.  (This information can be used to calculate the formula of the compound.)

Procedure

1. Clean a crucible and cover with tap water and then rinse with distilled water.  Place the crucible with cover on the clay triangle.  Make sure the cover is slightly askew so that water can escape when the crucible is heated.
2. After receiving directions from your instructor, heat the crucible and cover using a Bunsen burner for about five minutes.  Following this allow the crucible to cool for at least five minutes.  When the crucible is near room temperature, determine its mass.
3. Place between 1.0000 and l.5000 g of the hydrated compound in the crucible and again determine the mass.  Place the crucible with sample and cover on the clay triangle and heat for ten minutes.  Make sure that the cover is askew.  Allow the crucible and contents to cool for about five minutes and then re-determine the mass.  At this point you have all of the information needed to determine the percent water in the sample and the number of waters of hydration in the formula.

1. What is the name of the salt (not the hydrate part)?
2. Calculate the percent water in the hydrated sample.  Be sure the report the answer to the proper number of significant figures.
3. Calculate the moles of water and the moles of anhydrous compound in your sample.  Calculate the number of waters of hydration in the formula from this information.
4. Write the complete & correct formula for the hydrated compound you started with.  For examples, see the front of this sheet.
5. What effect would heating the hydrated sample for too short a time have on the calculated percent water?  Would the calculated percent water be lower or higher than the actual one?  Explain your answer.
6. Suppose the crucible and cover were not heated to dryness after being rinsed with distilled water.  Would the resulting determination of the percent water in the hydrate be lower or higher than the actual one?  Explain your answer.

Calculations & Wrap-up
Each student should pass in a clean, organized data table for this lab, and answer questions 1-6 above.  Show all your data in a data table, and show all your calculations with labels.
 

[MHS AP Chem page]