Determination of an Empirical Formula

General Chemistry - Summer 2001

Lab 2: 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, 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 roughly equivalent masses of zinc and iodine to yield a simple binary compound. The reaction is:

x Zn + ½ y I₂ ® Zn_xI_y

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 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. 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 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 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 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 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.

7) 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. 8) As the methanol is evaporating from the Erlenmeyer flask in step 7, also place the 250 mL beaker with the methanol solution on the hot plate. Do not let the methanol boil too vigorously. 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. Then remove from the heat, cover with a watch class and allow to cool to room temperature. Record the mass of the beaker. Data Analysis. The following analyses should be entered clearly in your lab notebook.

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. From your result, write a balanced equation that describes the chemical reaction that takes place between zinc and iodine. Finally, add the mass of the unreacted zinc to the mass of product to determine the percent of the total mass that is recovered by your procedure.

General Chemistry - Summer 2001 Name: ______________________

Lab #2 - Determination of an Empirical Formula

Mass of zinc used: _____________

Mass of iodine used: _____________

Mass of unreacted zinc: _______________

Mass of product: _______________

Total recovered mass: _______________

Percent Recovery: _______________

Moles of zinc consumed: _______________

Moles iodine consumed: _______________

Empirical Formula of Product: