EXPERIMENT 2
Stoichiometry
Introduction
Stoichiometry is the study of the quantitative relationships in chemical reactions. By studying
stoichiometry, you can calculate the quantity of reactants that will be consumed in a chemical reaction, and the
amount of product produced. Consider the reaction of vinegar with baking soda. As you may know, this reaction
produces carbon dioxide gas, which bubbles out of the vinegar. But if you want to know how much gas would be
produced from combining a teaspoon of baking soda with a cup of vinegar, you would need to consider the
stoichiometry of the reaction. Stoichiometry answers questions about chemical reactions dealing with “how much”
and “how many.”
In the first part of this experiment you will investigate the reaction of copper metal with oxygen in the air.
Using stoichiometry, you can predict the amount of copper oxide (CuO) that could be produced if all the copper
would react. However, when you perform the reaction yourself, you will discover that only some of the copper
reacts to form black copper oxide. A stoichiometric calculation will allow you to determine the amount of copper
oxide produced and the amount of unreacted copper remaining.
In the second part of the experiment, you will use the techniques learned in the first part to investigate the
chemical composition of an unknown compound. The solid compound will contain a certain quantity of water
trapped inside it, and your task will be to calculate the amount of water it contains. Based on the amount of water
released, you should be able to identify your unknown compound as one of the three possible compounds
described in the experiment. You will need to make stoichiometric calculations in order to find out how much
water should be released by each of the three unknown compounds, and compare that result with the amount of
water actually released by your compound.
Discussion
Investigation of a Copper Reaction
When heated in the presence of oxygen in the air, copper metal reacts to form copper oxide, CuO:
2 Cu (s) + O2 (g)
2 CuO (s)
Because only the surface of the copper metal will react to form copper oxide, you will use finely powdered
copper in order to maximize its surface area. You will heat the copper powder in a crucible, which is a small
thimble-shaped porcelain container used to heat substances to high temperatures. By heating the copper in a
crucible using an intense gas flame from a Bunsen burner, you will be able to make much of the copper react to
form copper oxide.
However, despite the use of fine copper powder, some of the copper will not be exposed to oxygen in the
air, and hence will remain unreacted. It is often the case that chemical reactions will not proceed entirely to
completion, and this copper reaction is an example of such a reaction. With such a reaction, the amount of product
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actually formed will be less than what could theoretically be produced in ideal circumstances. The amount of
product formed is often reported in terms of a percent yield. The percent yield for a given reaction is defined as:
precent yield =
mass of product formed
× 100 %
theoretical maximum mass of product
In this experiment, as the copper reacts to form copper oxide, the mass of the contents of the crucible will
increase. This mass increase will correspond to the mass of oxygen consumed during the reaction. A
stoichiometric calculation will enable you to determine the mass of copper oxide actually produced based on the
mass of oxygen in the final product.
The theoretical maximum mass of copper oxide can be calculated using stoichiometry based on the amount
of copper used in the reaction. By comparing the mass of copper oxide actually produced with this theoretical
amount, you can calculate the percent yield for your reaction. You would hope to get 100% yield in every
chemical reaction, but in reality a perfect yield is rarely attained.
Investigation of a Hydrated Salt
At one time, you may have seen simple humidity indicators that change color to indicate the amount of
water in the air. Or perhaps you have seen clothes that change color when wet, or children’s bath toys that behave
similarly. These items all depend on substances known as hydrated salts. A hydrated salt is a solid substance that
contains water bound within the solid. For instance, the natural mineral bieberite has the formula CoSO4·7H2O.
This means that, for every atom of cobalt in the solid, there are 7 molecules of water also “trapped” within the
solid. By heating the solid, the trapped water molecules can be released as water vapor:
CoSO4·7H2O (s)
CoSO4 (s) + 7 H2O (g)
Note that the resulting solid will weigh less due to the water lost in the process.
The release of the bound water is often accompanied by a color change. In the above example, the
hydrated salt CoSO4·7H2O is red-pink, while the anhydrous (“no water”) salt CoSO4 is dark blue. These types of
substances can be used to indicate the ambient humidity, because they will release water in dry environments and
absorb water in moist environments, changing color in the process.
In the second part of the experiment, you will be given one of the following hydrated salts, but will not be
told which one you have:
CuSO4·5H2O
CuCl2·2H2O
CoCl2·6H2O
Your task will be to remove all the water from your hydrated salt, determine how much water was
removed, and thus discover which of the unknown salts you were given. You will report the quantity of water
removed in terms of the ratio of moles of water released per mole of anhydrous salt. For instance, if your salt were
CuSO4·5H2O, you would expect to report that 5 moles of water were released per mole of anhydrous CuSO4. In
addition, you should observe any color changes during the course of the reaction. As described above, these color
changes will indicate the progress of the reaction.
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Use of the Crucible
The crucible is a thin, porcelain container designed to withstand high temperatures. We will be heating the
crucible using the gas flame of a Bunsen burner.
Place a ceramic triangle on a metal tripod or ring stand. Obtain a crucible from the center bench, making
sure it is not cracked, and place the crucible on the ceramic triangle. Connect a Bunsen burner to the gas outlet
with a rubber hose and have your TF come and verify your setup. Your TF will show you how to light the burner
and adjust the flame. The blue cone at the center of the burner flame should not be taller than one inch.
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TF: __________________
Name: __________________________
Experiment 2: Procedure, Lab Report and Prelab
Before You Come to Lab:
•
•
Read the entire lab report, including the previous introduction and discussion, and the entire procedure.
Complete the Prelab, which is the last page of the lab report, and turn in the prelab to your TF as you enter
the lab.
Safety in the Laboratory
•
•
•
•
•
Safety glasses and lab coats must be worn at all times while in the lab.
Nitrile gloves must be worn at all times while performing experiments or handling chemicals.
This lab involves working with fire, as such, long hair must be tied back and matches must be extinguished
properly under running water before being thrown away.
Be very cautious around the open flame of the Bunsen burner, and never leave a lit Bunsen burner
unattended.
Close the gas valve at the sink in order to turn off the Bunsen burner. Be sure the gas valve is closed
whenever your burner is not lit.
Waste Disposal and Cleanup
•
•
All waste produced in this experiment will be solid waste. Solid waste must be kept dry in order to
dispose of it properly. DO NOT try to rinse or wash out the crucible.
Place your used crucibles, with their contents, in the collection bin in the waste hood. Do not dispose of
the crucibles or any chemicals in the trash, and do not attempt to wash out the crucibles.
Before You Leave the Lab
•
•
•
Have your TF check your lab bench for cleanup.
Submit your data and lab report to your TF. This page and all subsequent pages must be stapled and
turned in.
Wash your hands before leaving the lab.
Grading:
Prelab:
_____ / 10
Lab Report:
_____ / 20
Safety:
_____ / 3
Cleanup:
_____ / 2
Total:
_____ / 35
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TF: __________________
Name: __________________________
Investigation of a Copper Reaction
Data, Observations, and Notes
First, take a clean, dry crucible from the center
lab bench, bring it to the balance, and weigh the empty
crucible. Using the analytical balance, weigh out
approximately 1 gram of the copper powder. Add this
powder to the empty crucible, and weigh the filled
crucible. Return the crucible to the burner flame, and
heat the copper powder. Do not heat the crucible so
intensely that it glows red-hot. Periodically remove
the crucible from the flame and gently tap it against
the bench top in order to mix up the copper. Be
careful not to let some of the copper spill out of the
crucible, or your final mass will be inaccurate. While
the crucible is away from the flame, note the color of
the contents. Continue heating for at least 25 minutes,
or until no more color change is observed.
Mass of empty crucible:
Using the crucible tongs, carefully set the
crucible aside to cool. When it has reached room
temperature, weigh the crucible and its contents on an
analytical balance.
Final mass of crucible with product:
Mass:
Mass of crucible with copper before heating:
Mass:
Color changes observed:
Mass:
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TF: __________________
Name: __________________________
Investigation of a Hydrated Salt
Data, Observations, and Notes
This part of the experiment is performed very
much like the copper reaction. In your prelab report,
you will have to write out a detailed procedure. Your
task will be to determine the mass of water released by
heating one of the following hydrated salts:
CuSO4·5H2O, CuCl2·2H2O, CoCl2·6H2O. You will not
know which of the salts you are heating, but should be
able to deduce this from your final calculations.
As part of your prelab, write up an appropriate data
table in the space below that you can use while
performing the laboratory experiment. Don’t forget to
leave space in which to note the observed color changes.
Here are some helpful hints for designing a
successful procedure:
•
Heat the compounds very gently. If the
compound starts to turn brownish-black, gives
off green gases, or starts to bubble and boil,
then it is being overheated.
•
Record all color changes observed for the
compounds as they are heated. One of the
compounds should exhibit more than one color
change due to the formation of an intermediate
stable hydrate with 2 moles of water per mole
of anhydrous salt.
•
The colors of the three anhydrous salts, in no
particular order, are light blue, greenish-white,
and yellow-brown. Once your entire sample
has reached one of these colors, you should
stop heating in order to avoid overheating.
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TF: __________________
Name: __________________________
Lab Report
Results
1. Determine the mass of CuO that was produced in the copper reaction, and calculate the percent yield of this
reaction.
Mass of CuO:
Percent yield:
2. Calculate the mass of water released from the hydrated salt.
Mass of water:
3. Determine the ratio of moles of water to moles of anhydrous salt assuming the unknown salt is CuSO4. Leave
ratios as decimals.
Ratio:
Repeat this calculation assuming the salt is CuCl2. Ratio:
Repeat this calculation assuming the salt is CoCl2.
Ratio:
4. What is the most likely identity of your hydrated salt?
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TF: __________________
Name: __________________________
Prelab
1. Provide a brief procedure for determining the mass of water released upon heating a hydrated salt. Be sure you
write up a data table in the appropriate space on that page of the experiment.
2. One student weighed 1.000 gram of copper powder into a crucible and heated it to produce copper oxide. He
was able to obtain only a 15% yield. What was the total mass of the crucible contents (CuO plus unreacted Cu)
after the reaction?
Mass of contents:
3. Another student heated 1.000 gram of a red-pink hydrated salt until all the water had been removed. She was
left with 0.546 grams of a dark blue solid. Is it possible that the hydrated salt was bieberite, CoSO4·7H2O? Why
or why not? Calculations should be shown.
4. Your lab partner removes their goggles and an unknown chemical accidently splashes into their face. What did
your lab partner do wrong? What should you do to help them? Be precise as to the order of events.
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TF: __________________
Name: __________________________
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