Knutsen's Chem 140 Lab: Formula of a Hydrate

Chem& 161
General Chemistry w/Lab I

Experiment: Formula of a Hydrate

Purpose
In this laboratory you will determine the percent water in an unknown hydrate, and use that information to determine the formula of the hydrate. You will receive an additional grade dependent on your accuracy in determining the percent water, as explained in the course syllabus.

Introduction
Hydrates are ionic compounds that contain water as part of their crystal structure. The percent by mass of water in a hydrate is a fixed quantity and can be determined through a simple experiment.

You will be given a sample of a hydrate, and are to determine the percent water in the compound and the complete chemical formula. For example, you might receive the hydrate of copper (II) sulfate. The formula for copper (II) sulfate is CuSO₄•xH₂O, where x represents the number of moles of water per mole of copper sulfate. By heating the compound and driving off the water, you will be able to determine the percent water, and from that the correct formula for the compound.

Procedure
In this lab you will work individually on your own unknown sample, and will be responsible for determining its percent water and correct chemical formula. However, even though you are working alone, you are encouraged to confer with others at your bench in order to determine the best methods and procedures to use, and may draw on any previous experience/resources you are familiar with to devise your procedures.

You are to devise your own procedure for determining the percent water in your assigned hydrate. The following points are some considerations to keep in mind as you go about devising a procedure. It will help to discuss these questions with others at your lab bench before proceeding.

How much of the unknown should you use? What are the advantages and/or disadvantages of using a large sample? A small sample?
How many times should you repeat the experiment?
How can you be certain that all of the water has been driven off?
What can you do to prevent the compound from reabsorbing water after it has been heated?

Notes:

Each unknown will have a number on it. Be sure to record this number in your lab notebook.
The amount of the unknown you receive will be all you will have to work with. Plan accordingly!
NEVER weigh objects while they are hot. This will give an incorrect reading, and may also damage the scale.

Analysis and Calculations
Calculate the percent water in your assigned compound. Once you have calculated the percent water, see the instructor to check the accuracy of your results. Remember, you will receive an additional grade for the lab based on the accuracy of your results! (See the syllabus for additional details.) If you are unsatisfied with your results you may repeat the lab on your own time. However, any such make–ups must be scheduled in advance with both the instructor and the lab technician.

After your results have been approved, the instructor will give you the chemical formula for the anhydrous form of the compound. Use this information, and your experimental percent composition, to calculate the experimental formula of the hydrate. For example, your unknown may be copper (II) sulfate, CuSO₄•xH₂O. Your task is to determine the value of x, which is the number of moles of water per mole of copper sulfate. Note that this value will almost certainly not be a whole number.

Then use the theoretical percent composition, supplied by your instructor, to calculate the actual formula of the hydrate. Be sure to discuss the two formulas, experimental and actual, in your analysis.

As a part of your analysis you will be expected to calculate the percent error in your experimental results, and to discuss the error in your write–up.

Return to the Laboratory Experiments Schedule