Chemistry 1 – Zimmer
Prelab due Thur 5/1
Full report draft due 5/2
Final draft due Monday 5/5
Name:
LAB: EMPIRICAL FORMULA OF ZINC IODIDE*
Introduction and Purpose:
We will cap off our units on ionic and covalent compounds, moles, and solutions by
carrying out an experiment that combines all these topics. The purpose of the
experiment is to answer a question very typical to the study of chemistry: what is this
substance I’ve created? This lab will also introduce the subject of chemical reactions,
which will become a major focus of study next year in Chem 2.
You will carry out a chemical reaction to make the ionic compound zinc iodide from its
elements. Since zinc is a transition metal, its charge in ionic compounds is not
necessarily predictable, but you will be able to use mass analysis to calculate the
composition of the compound you make and find its formula. You’ll be comparing
your result to the actual values to determine percent error of your result and analyze the
sources of error.
Prelab Questions:
1. The elements zinc and iodine can react with each other to form an ionic compound.
Explain in as much detail as possible what happens to the atoms of zinc and iodine
as this reaction occurs.
2. Why do zinc and iodine atoms undergo opposite changes during this reaction? Put
your answer in terms of their differences in atomic structure and use the concept of
electrostatic attractions and repulsions.
3. Using your textbook or the Internet (Wikipedia or webelements.com are OK sources),
fill out a table like the one below about the properties of our reactants IN YOUR LAB
NOTEBOOK.
Reactant
Formula Appearance Solubility in water Safety issues
Zinc (granules)
Iodine (crystals)
Adapted from “Synthesis and Decomposition of Zinc Iodide,” Chem085 online pre-lab
tutorial, Southeast Missouri State University.
*
4. After reading carefully through the procedure, list the materials you’ll need for this
experiment.
5. Make a data table with labeled spaces for all measurements you’ll make during the
experiment on a separate piece of paper, and attach it to this handout.
Procedure: Carefully read through these instructions BEFORE beginning the lab, so that
you have an overview of the procedure and can create a useful data table for recording
your observations and measurements. Measurements you need to make are in bold.
After some of the steps, there are questions in italics. You should discuss these with
your lab partner as you carry out the experiment, and make note of your thoughts in your
lab notebook. (Some of these questions will be easier to answer after completing the
procedure.) Your answers will help you write a better, more complete discussion for your
report.
1. Measure and record the mass of an empty medium test tube.
2. Weigh out approximately 2.0 g of zinc granules into your medium test tube, and
record the precise combined mass. Why doesn’t it matter if you get exactly 2.0 g?
3. Add approximately 2.0 g of iodine crystals to the same tube, and record the precise
combined mass. NOTE: iodine vapor is dangerous – do not inhale and be
careful not to touch the crystals with your skin. Observe and record the
characteristics of the reactants, including any changes that occur when they are
combined.
4. Carefully add 5 ml of acidified water to your reaction test tube. Mix the reactants by
strongly tapping the test tube with your finger. You must vigorously mix the reactants
during the course of the reaction in order for it to finish in a reasonable time frame
(about 10 minutes). Observe and record any changes to the mixture, including
changes in phase, color, and temperature, with approximate times indicated for each
change that occurs. What happens to the reactants when the water is added? Why is
this important?
5. While one partner continues to mix the reaction, the other partner should measure
and record the mass of the large test tube with two boiling chips.
6. Continue to mix and observe the reaction until it is complete. At this point, carefully
decant the solution into the large test tube without disturbing the solid at the bottom.
In order to transfer all of the dissolved product, rinse the solid zinc 3 times with about
1 ml of acidified water for each rinse, decanting each rinse into the large test tube.
How do you know when the reaction is complete? What chemical species are in both
test tubes when the reaction is complete? Why do you need to rinse the zinc and add
rinse water to your large tube?
7. In order to wash the solid zinc of any remaining impurities, rinse it 3 more times with
1 ml of acidified water, decanting each wash into the sink. Why do you rinse the zinc
even more, but get rid of the water?
8. One partner should gently dry the remaining zinc granules by carefully passing the
test tube through a Bunsen burner flame until the moisture starts to boil away. When
the zinc is completely dry, the granules will not stick to each other or the sides of the
tube. Once the zinc is dry and cool, measure and record the mass of the test tube
with leftover zinc. Why is it important to remove all water from the leftover zinc?
Why is it important to wait till the tube has cooled before measuring its mass again?
9. The other partner should gently heat the test tube of dissolved zinc iodide in order to
evaporate the water and isolate the zinc iodide product. After a few minutes of gentle
heating, the water should boil off leaving a white to off-white product. As the zinc
iodide dries, it will make a cracking noise; once the cracking stops, the product is dry.
If you heat the zinc iodide too strongly, it will decompose and change colors (from
white to dark yellow or purple). Also make sure to heat the sides of the test tube so
that no vapor or water drops remain inside the test tube. After the product is dry and
has cooled just enough to touch, measure and record the combined mass of the
test tube with boiling chips and zinc iodide. What happens to the ions of zinc and
iodide when you evaporate the water? Why is it important to make sure there is no
condensation around the top of the tube?
10. Dispose of the leftover zinc in the “recycled Zn” container, and rinse out the zinc
iodide product in the sink. Try to save the boiling chips if possible. Please also wash
out all test tubes and leave them upside-down in the racks to dry.
Calculations and Results:
These calculations should be done neatly in your lab notebook. Use BOTH word
equations and values with units, and label each calculation, so that both you and I can
follow your work. Complete the following calculations BEFORE CLASS FRIDAY:
 masses of zinc and iodine that reacted
 mass of zinc iodide product
 experimental percent composition of zinc iodide produced
 empirical formula of zinc iodide produced
After our class discussion on Friday, when I will tell you the actual formula, complete
these calculations as well:
 theoretical percent composition of zinc iodide
 error in your experimental %Zn value as compared to theoretical value
Procedure and error analysis questions:
The following questions will be discussed in class following the experiment, as time
permits. You can take notes during this discussion, which will be helpful when you
complete your lab report at home. After class, type (at least 1.5 spaced) answers to each
of the following questions, in paragraph form, and attach them to your notebook pages.
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What would have happened differently if you had not added the water to the
reactants? Why?
Why did all the iodine get used up during the reaction, even though there was still
zinc left over? How could you tell Iodine was the LR (limiting reactant)?
Assuming you used the same balance for all mass measurements, your
measurements all have the same level of precision. Which specific measurement do
you believe was least likely to be accurate, and why?
How precise was the class’s data? (How large was the range between the highest
and lowest values for percent zinc?) If your result was not close to the class average
(meaning it was an outlier), explain why.
Compare your calculated % Zn to the theoretical % Zn to determine the percent error
of your result. (Make sure you indicate whether your % Zn was higher or lower than
the theoretical value.
The following is a list of potential sources of experimental error. For each one, explain
whether the source of error would result in your calculated % Zn appearing too high
or too low, or have no effect on the calculated % Zn.
o Adding a little too much or too little acidified water
o Not waiting long enough for the reaction to complete
o Letting a small piece of zinc get into the 2nd (large) test tube while decanting
o Not sufficiently rinsing the zinc with acidified water after the reaction
o Not drying the leftover zinc long enough
o Letting water vapor remain on the sides of the test tube after heating the tube
containing zinc iodide