Determining the Hydrate

Formula of an Ionic Compound

Introduction

You will be using Bunsen burners and ceramic crucibles to remove the water from a hydrated ionic compound. Of what use are dehydrated ionic compounds? During the course of this lab you will be working with very hot ceramics. It is crucial to remember that ceramics remain dangerously hot long after you stop heating them! Pay close attention to your instructor when proper safety precautions are demonstrated.

Materials

The following materials will be supplied for this lab:

Copper II sulfate

Cobalt II chloride

Safety goggles

Electric balances

Ceramic crucible ($7.00)

The following materials will be needed out of your equipment drawer:

Bunsen burner and sparker

Large ring, ring stand

Ceramic triangle

Crucible Tongs

Scooper

Procedure

1) Check your crucible carefully for cracks or chips. If any are present, trade the crucible with your instructor for an undamaged one. Discoloration inside your crucible is harmless, and will not effect your results.

2) Wash and dry the crucible thoroughly. Place them in the ceramic triangle, above the Bunsen burner, as illustrated here. Heat the crucible to redness and allow to cool. Keep the crucible in the triangle during all cooling steps.

3) Take your crucible to the balance. Be sure the balance has been zeroed before every mass determination! Weigh the crucible and record the mass. Add approximately 2 grams of the copper sulfate to the crucible and record the new mass.

4) Heat the crucible with the copper sulfate gently for three minutes. You may hear a sizzling sound as the water leaves the crystals and evaporates off of the hot ceramic. Be sure not to scorch the copper sulfate! What color does the anhydrous salt become?

5) When all water has been evaporated, heat a little more strongly, being careful not to scorch the anhydrous salt. When the color change is complete, allow the crucible to cool.

6) Get the final mass of the anhydrous salt and the crucible. Did it get lighter or heavier?

7) Add a few drops of water to the anhydrous salt. What happens? Place the salt in the proper container in the fume hood. Read the labels! Don't mix them up!

8) Repeat steps 3-8 for cobalt II chloride. Be sure to wash and dry the crucible again. Be especially careful during step 4. It may be useful to hold the Bunsen burner by the base and waft the flame under the crucible.

Data Analysis

a) Determine the mass of the hydrated copper II sulfate.

b) Determine the mass of the anhydrous copper II sulfate. How many moles of anhydrous copper II sulfate is this?

c) Determine the mass of water lost. How many moles of water is this?

d) Determine the ratio of water moles to copper II sulfate moles. Round your answer to the nearest whole number.

e) Use the value from (d) to write the formula for the hydrated salt. Repeat these calculations for the cobalt II chloride.

Results and Conclusions

In your conclusion, you must address the following:

a) What are the major sources of error in this experiment? Remember, you cannot include human error (miscalculations, dropped crucibles, scorched salts, etc).

b) What safety cautions would you emphasize for those who next perform this lab?

c) What are your formulas for the hydrated salts?

d) Imagine you are performing this lab on a humid day. How might that effect your results?