Chemistry 1 Lab
Fall 2008
Lab #3: Determination of a Chemical Formula: The Reaction Between Iodine and Zinc*
*Adapted from "Chemistry in the Laboratory", J. L. Roberts, J. L Hollenberg and J. M. Postma, 4th ed., W. H. Freeman, 1997.
September 17, 2008
Introduction. When attempting to understand the properties of, or predicting the reactivity of, a
given material, perhaps no piece of information is more critical its composition. Therefore, when
chemists either make or discover new materials, one of the first steps in their work is to go
about determining (or confirming) its chemical composition, specifically what elements are
present and in what amounts. For many compounds this is done by combustion analysis, a
technique whereby the elemental composition of a material is ascertained by quantitative
analysis of its combustion products. Another way of finding the composition of a compound is to
synthesize it and, in the process, measure the masses of the various reactants that are
consumed. This will be the strategy you use in this lab when you determine the empirical
formula of zinc iodide. Specifically, you will react roughly equivalent masses of zinc and iodine
to yield a simple binary compound, ZnxIy.
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 unreacted. By measuring the
mass of zinc that remains, the mass of zinc that was consumed can be determined. At that point
you will have enough information to calculate the empirical formula for the product. Finally, you
will 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. Methanol is flammable - do not use near open flame. Methanol is also toxic;
use in the fume hoods and avoid breathing vapors.
Procedure:
Part A: The Reaction of Iodine with Zinc.
1) Weigh a clean, dry 125 mL Erlenmeyer flask (to the nearest 0.01 g). You will need this
mass determine the mass of unreacted zinc.
2) Weigh about 2 g (2.0 ± 0.1 g; weigh to the nearest 0.01 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 0.01 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 methanol 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. When the reaction is
complete (how will you know?), 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
methanol to the flask and swirl for 15 seconds. Pour the methanol into the beaker. Repeat
the washing of the unreacted zinc with another 5 mL and add to the beaker.
6) Place the Erlenmeyer flask back on the hot plate to remove all of the remaining methanol.
Increase the temperature of the hot plate to a medium setting. Continue heating until all of
the methanol has evaporated; the zinc granules should be dry and flow freely in the flask.
Remove the flask from the heat and allow to cool to room temperature. Record the mass of
the flask.
Part B: Isolation of Reaction Product.
7) As the methanol is evaporating from the Erlenmeyer flask in step 6, also place the 250
mL beaker with the methanol solution of your reaction product on the hot plate. Do not let
the methanol boil too vigorously or you will lose product from spattering. It will take between
20 and 30 minutes to remove the methanol completely. After the last trace of methanol has
evaporated, continue heating for 2 minutes.
8) Remove the beaker from the hot plate and place in a desiccator for about 15 minutes to
cool to room temperature. Once it is cool, remove the beaker and measure its
mass. Describe its appearance in your notebook – how does it differ from the starting
materials?
Data Analysis. Using your mass measurements, determine the composition of the reaction
product as a mass percent (e.g., percent mass zinc and percent mass iodine). Then determine
the empirical formula of the reaction product. Now, despite the fact that the textbook definition of
empirical formula is the smallest whole number ratio of the atoms in a compound, you will not
attempt to find such a ratio. Why? Because experimental error will make it highly unlikely that
your results can be expressed so simply. Instead, calculate the number of moles of zinc and
iodide and then divide each by the smaller number. For instance, the formula could be
expressed as ZnaIb, where a and b are the number of moles of zinc and iodine in the product,
respectively. For the sake of argument, let’s say that a > b. Dividing both numbers by b allows
you to write the formula as Zna/bI without changing the ratio of atoms in the compound.
When you have calculated the ratio of iodine to zinc atoms in the empirical formula pool your
results with the rest of the class. Calculate the mean, standard deviation, and 95% confidence
interval of y, the iodine subscript in the empirical formula ZnxIy. Does the 95% confidence
interval for the iodine include a "simple" integer? Based on the class analysis and your own
results, what is the "most likely" empirical formula for the reaction product?