Chemistry 140

Please have parts 1, 2, 3 and 4 (tables only) ready before class on Tuesday, October 15 (the lab is the next day). Write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up are due in class on Wednesday, October 23.

This is the standard “lab manual” format for a lab. It will be your job to translate it into the familiar “parts” of the report expected in Chem 140 (some guidelines are given at the end of the lab protocol below).

Lab 2: Determination of the empirical formula of the product of magnesium heating

In this experiment, you will determine with which component of air (nitrogen or oxygen) magnesium will react when heated, by identifying the percent composition of the product of that reaction.

Air is composed of 78% nitrogen (in the form of N2) and 21% oxygen (in the form of O2), with a trace of other gases. When a strip (or ribbon) of magnesium is heated in a porcelain crucible, it emits an intense white light for a few minutes and a gray-white powdery residue remains. The experiment will help you determine the identity of that powdery residue.

One possibility is that magnesium metal simply oxidizes by heating it in air:

2 Mg(s) + O2 (g) ® 2 MgO(s)

The other possibility is that magnesium reacts with the nitrogen in the air to form magnesium nitride:

3 Mg(s) + N2 (g) ® Mg3N2(s)

By determining the % Mg in your product, you can identify the product as MgO or Mg3N2. From the mass of magnesium before heating and the mass of the product, you can calculate the experimentally determined percent Mg in the product:

Materials:

Magnesium (Mg)

Crucible (with cover)

Bunsen burner

Matches

Tongs

Metal ring

Clay triangle

Wire gauze

Digital balance

Procedure: Work in groups of 2.

0. Figure out where the equipment for the lab is kept (some of it will be in the cabinet, some of it will be on the cart).

1. Inspect the crucible for cracks. If your crucible is cracked, exchange it for a new one. Clean the crucible and cover thoroughly. The crucible may be stained, so you will not be able to get it completely white. After cleaning be sure to handle it ONLY with metal tongs.

2. Place the clay triangle on an iron ring clamped to a ringstand. Using tongs, transfer the crucible and its cover to the clay triangle. See if you can identify the materials list above in the picture.

Use the striker to light the Bunsen burner.

Begin heating them with the Bunsen burner. Adjust the burner to obtain a blue flame having an inner lighter blue cone. The tip of the cone is the hottest part of the flame; use this part of the flame to heat the crucible red hot.

3. After heating the crucible, cool it in place on the ringstand for about five minutes to room temperature.

4. Using the digital balance, weigh the crucible and cover to the nearest milligram (.001g) and record the mass in your notebook. Place the magnesium sample into the crucible and then weigh the crucible, cover and sample. Record this mass and the physical appearance of the magnesium metal in your notebook. Notice that you can't merely tare the crucible and cover.

5. Cover the crucible with its lid and place it inside the clay triangle supported by a ring and ringstand (see demo model). Heat GENTLY. After 10 minutes check to make sure that most of the metal has reacted by using tongs to lift the lid SLIGHTLY. If the metal is burning, do not look directly at it, but replace the lid and continue heating gently until flames are no longer visible.

6. Remove the lid and heat the crucible with the hottest part of the flame for 5 minutes. Record the physical appearance of the residue.

7. When the crucible and its contents have cooled (after five minutes), weigh them to the nearest milligram and record this mass in your notebook. Use the same digital balance you used for the previous massing.

8. Dispose of the magnesium product in the container designated for waste disposal. Clean the crucible and cover.

9. Repeat the experiment for a total of three trials.

Sample Data Sheet for magnesium experiment:

Mass of empty crucible and cover ___________________

Mass of crucible, cover and Mg ___________________

Mass of Mg ___________________

Mass of crucible, cover, and product ___________________

Mass of product ___________________

Results:

theoretical % Mg in MgO ___________________

theoretical % Mg in Mg3N2 ___________________

experimental % Mg in product ___________________

To calculate the theoretical % Mg in a compound, calculate the mass of Mg in one formula weight of the compound, then divide by the formula weight (or molar mass) of the compound.

And now for the hints on how to set up your lab book:

Your name, your partner’s name, date of experiment

Lab 2: Determination of the empirical formula of the product of magnesium heating

Part 1. Purpose

Please write a sentence or two stating the goal of this experiment, based on the information above.

Part 2. Materials and methods

Chemicals needed: magnesium metal strip

Examine the setup shown above, draw the equipment shown and label the various pieces of equipment.

Part 3. Procedure

There should be no major modifications to the procedure above.

Waste disposal — Dispose of any magnesium-containing materials in the container designated for magnesium waste disposal in the hood.

Part 4. Original data

Perhaps the sample data sheet shown above could be converted into a table for the three trials you will doing.

Part 5. Calculated results

Note that you should include your calculations for both of the theoretical magnesium percent compositions as well as your experimental magnesium percent composition (one trial’s worth will do; I trust your other two trials’ calculations will be done in an identical manner!). Organize your calculated results into a table.

Part 6. Group results

Write all three of your experimental magnesium percent compositions on the board and record all groups' experimental magnesium percent compositions. Calculate the mean and standard deviation, and comment about where your results fit in (for instance, are your results outliers?).

Part 7. Questions

1. Why can't you simply tare the crucible and cover when recording masses in step 4?

2. From your results, is your product MgO or Mg3N2? Looking over the results of your three trials, did you have a systematic error? In any case, identify the sources of error in your experiment and how you would expect the error to affect your results.

3. Compare the appearance of magnesium metal with the product after heating. In what way could you tell that a chemical reaction occurred?

4. After looking at the group results from part 6, are you more or less confident of your answer to question 2? Mention how the standard deviation can be used in this context. Did the class as a whole suffer from a systematic error? Were there any outlier values which, if thrown out, improve the standard deviation dramatically?

Part 8. Conclusion

The conclusion should be a brief recap of your results and sources of error (confidence you have in your results).

First sentence: “We determined that the magnesium percent composition of our material was _________, __________ , and __________ in three trials, which resulted in a mean value of ____________ ± _____________ %.”

Second sentence: “From these results, we concluded the empirical formula of the product of heating magnesium in air is ___________________. because ______________________________ .”

Next couple of sentences: Mention any significant random and/or systematic errors and how they could be fixed or not fixed next time.

Next sentence: “The class mean was _________ ± _________ %, which is (consistent/inconsistent) with our result.”

Last sentences: Did you accomplish the goal of the experiment? How confident are you of your results?

Abstract

Again, a short (less than 100 word) summary of the major result(s) of your experiment, and the method by which you achieved this result.

Name, school affiliation, title, text as usual. Keep in mind: What was the major result (and uncertainty)? By what method did you achieve the result?