Water in a Hydrate
Background Information
Some ionic salts bind to and trap water molecules in their crystal
structure. These compounds are referred to as hydrates. The process is not
random; a particular hydrate will bind to the same number of water molecules
each time. For example, sodium carbonate will bind with ten water molecules
to form a crystal called sodium carbonate decahydrate (Na2CO3•10H2O).
Hydrates can be either hydrous (containing water molecules) or anhydrous
(without water). Typically, the water molecules can be driven off by moderate
heating over a Bunsen burner. Heating the sodium carbonate decahydrate
would result in the loss of ten water molecules per sodium carbonate.
Often times the hydrous version is a distinctly different color than its
anhydrous counterpart. In today’s lab, you will be working with copper(II)
sulfate. Hydrous copper(II) sulfate has a bright blue color whereas the
anhydrous form has a chalky gray color. The addition of a few drops of water
turns anhydrous copper(II) sulfate a bright blue color.
Question:
How many water molecules are attached to each copper(II) sulfate?
Pre-Lab Questions:
1. Will grams or moles be more helpful in answering the question for this
lab? Explain your reasoning.
2. In order to calculate the number of moles of something, what two pieces
of information do you need?
3. What will happen to the mass of the copper(II) sulfate hydrate as it is
heated. Explain your reasoning.
a. How will this help you in determining the number of water molecules
attached to each?
Procedure:
1. Devise a procedure for answering the question above. You will have to
call upon your knowledge of grams and moles to answer the question.
The following information will help you plan.
a. Overheating the copper(II) sulfate will cause the compound to
decompose. The decomposition can be observed by the distinct
reddish-brown color of copper.
b. During heating, you will need to stir the hydrated copper(II) sulfate to
prevent it from clumping.
c. If you use a crucible, be sure to clean it our first and then heat over Page 1
the flame to drive off all excess water, wiping it dry is not enough.
d. When working with Bunsen burners, be sure to have hair pulled back
and loose clothing secured.
2. Record your procedure in your lab notebook. Be sure to include a data
table.
3. Get your procedure approved prior to collecting data.
Data Analysis:
After collecting data, make a new section in you lab titled Data Analysis.
Introduce any calculations with a sentence describing what the calculation is.
For example, a calculation of percent yield would look as follows.
We used the final mass of barium chloride and the mass we
calculated from the balanced equation to find the percent yield.
23.6gBaCl
2
26.8gBaCl
100% = 88.1%
2
Post Lab Questions:
Write a conclusion describing in detail how you determined the number of
water molecules attached to each copper(II) sulfate. Include the final formula
for the hydrous form of copper(II) sulfate. If you need help writing the
formula for a hydrate there is an example in the background information of
the formula for sodium carbonate decahydrate.