AP Chemistry
Stoichiometric Determination: Reaction Between Zinc and
Iodine*
* Adapted from "Chemistry in the Laboratory", J. L. Roberts, J. L Hollenberg and J. M. Postma, 4 th ed., W. H.
Freeman, 1997.
Purpose:
The goal of this experiment is to determine the empirical formula of the compound formed
between iodine and zinc. You will also balance a chemical equation that describes this reaction.
This is also an exercise in recording qualitative observations concerning a chemical reaction.
Introduction:
In this experiment, you will react together equivalent masses of zinc and iodine to yield a simple
binary compound. The reaction is:
__Zn + __ I2 ___ ZnxIy
Zinc is a reasonably active metallic substance that will readily react with any of the Halogens.
Iodine is a shiny purple crystalline solid that is typically sold as an alcoholic tincture. These
elements, Atomic Number 30 and 53 respectively, will react to form a white powdery solid. This
solid, zinc iodide, is very hydroscopic (absorbs water from air) and is very soluble in water.
The concept of a limiting reactant is very important in the study of the stoichiometry of chemical
reactions. The limiting reactant is the reactant that controls the amount of product possible for a
process, because once the limiting reactant has been consumed, not further reaction can occur.
You will determine the value of coefficients x and y by carefully measuring the masses of zinc
and iodine used in the reaction. As it turns out, all of the iodine will be consumed (this is how
you will know when the reaction is complete - the dark color of the iodine will fade as it is
consumed during the reaction) and some of the zinc will remain un-reacted. By measuring the
mass of zinc that remains, the mass of zinc that is consumed can be determined. Then, the mass
of both the zinc and iodine in the product can be used to determine the empirical formula of the
product.
You will also check the mass balance of the reaction to verify that mass is conserved in the
conversion of reactants to products.
Safety Notes: Handle solid iodine and solutions of iodine carefully - they can cause skin
irritation or burns. Alcohol is flammable - do not use near open flame. Alcohol is also toxic so
try to avoid breathing vapors.
Pre-Laboratory Questions:
1. Calculate the theoretical percentage by mass of zinc and iodine in zinc iodide, ZnI2.
2. Suppose 2.033 g of iodine is completely reacted with zinc. What is the theoretical
amount of zinc iodide that should be produced?
Material:
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
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Granulated zinc
Iodine crystals
125 mL Erlenmeyer flask
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AP Chemistry

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Ethyl-Methyl alcohol mixture (40 mL per group)
Aluminum foil
Hot plate
250 mL beaker
Procedure:
Part A: The Reaction of Iodine with Zinc.
1. Weigh a clean, dry 125 mL Erlenmeyer flask as precisely as possible (to the nearest
0.001 or 0.0001 g). You will need this mass in step 7 to determine the mass of unreacted
zinc.
2. Weigh about 2 g (2.0 ± 0.1 g; weigh to the nearest 0.0001 g) of granulated zinc. Record
the mass and add the zinc to the Erlenmeyer flask.
3. Weigh about 2 g (2.0 ± 0.1 g; weigh to the nearest 0.0001 g) of iodine. Record the mass
and add the iodine to the Erlenmeyer flask.
Note: Iodine is corrosive to stainless steel - clean up any spilled iodine that you may get on
the balances.
4. In a fume hood, add 25 mL of ethyl-methyl alcohol to the mixture. Cover the flask with
aluminum foil. Place the flask on a hot plate and turn the hot plate on a low setting.
Record the physical appearance of the mixture. Continue gently boiling until the dark
color of the iodine fades; swirl occasionally to dissolve the iodine.
5. As the reaction progresses, weigh a clean, dry 250 mL beaker for use in step 6.
6. When the reaction is complete, remove the Erlenmeyer flask from the hot plate (be
careful! - use paper towels to prevent burns). Pour the liquid from the flask into the
weighed beaker. Be careful not to transfer any of the unreacted zinc granules. Add an
additional 5 mL of ethyl-methyl alcohol to the flask and swirl for 15 seconds. Pour the
ethyl-methyl alcohol into the beaker. Repeat the washing of the unreacted zinc with
another 5 mL and add to the beaker.
7. Place the Erlenmeyer flask back on the hot plate to remove all of the remaining ethylmethyl alcohol. Increase the temperature of the hot plate to a medium setting. Continue
heating until all of the ethyl-methyl alcohol has evaporated; the zinc granules should be
dry and flow freely in the flask. Remove the flask from the heat and allow cooling to
room temperature. Record the mass of the flask.
Part B: Isolation of Reaction Product.
8. As the ethyl-methyl alcohol is evaporating from the Erlenmeyer flask in step 7, also place
the 250 mL beaker with the ethyl-methyl alcohol solution on the hot plate. Do not let the
ethyl-methyl alcohol boil too vigorously. It will take between 20 and 30 minutes to
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AP Chemistry
remove the ethyl-methyl alcohol completely. After the last trace of ethyl-methyl alcohol
has evaporated, continue heating for 2 minutes. Then remove from the heat, cover with a
watch class and allow cooling to room temperature. Record the mass of the beaker.
Data Analysis: Determination of an Empirical Formula
1.
2.
3.
4.
5.
6.
7.
8.
9.
Mass of zinc obtained: _____________
Mass of iodine obtained: _____________
Mass of un-reacted zinc: _______________
Mass of product collected: _______________
Moles of zinc consumed: _______________
Moles iodine consumed: _______________
Calculate the theoretical yield by taking the mass of your zinc that reacted.
Calculate your Percent Error: _______________
Using your mass measurements, determine the percent composition by mass of the
reaction product (percent composition of the zinc and percent composition of iodine):
10. Empirical Formula of Product:
11. Write a balanced equation that describes the reaction that takes place between zinc and
iodine:
12. Using your balanced equation from (11) and the mass of iodine, calculate the amount of
product you would theoretically make.
13. How close is your real mass from the theoretical one?
14. Using your mass of product calculated in (12) calculate the amount of Zinc necessary to
have given you your theoretical amount of product.
15. How close is your real mass of Zinc used from the theoretical one?
Post-Lab Questions
1. Use an appropriate reference to determine the melting points of zinc, iodine, and zinc
iodide. Explain why they are different.
2. How would the percent Zn change if some product is lost during the washings? Be
specific.
3. Why have we capped the flask with aluminum foil during the reaction? Why was a
rubber stopper not simply used to cap the flask?
4. Why was the reaction mixture heated?
Conclusion
Here are items to think about when you write the conclusion. Don’t just answer these
questions, but use them to formula a thoughtful conclusion based on the purpose of the lab.
What did you conclude about the empirical formula for zinc iodide and does it make sense
based on the valences for both zinc and iodine? What would cause your recovered mass to
be different from what is should have been and how did that affect the outcome of the
experiment. What errors did you have in the experiment that affected the outcome and how
did they affect it?
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Revised: 28 August 2009