Chemistry 101

Lab 3: The empirical formula of an oxide of magnesium

Due date: Beginning of class, Thursday, July 19.

Objective: Determine the empirical formula of an oxide of magnesium by combustion analysis.

Prelab: On the prelab sheet provided, calculate the empirical formula of an oxide of magnesium based on the sample data given on the sheet.

Remember: Use the same balance for the entire lab; some of your grade on this lab will be for the accuracy of your empirical formula.

Procedure:

1. Record the mass of a dry crucible with its cover.

2. Get a shiny piece of magnesium ribbon from the instructor (use the steel wool to remove excessively dull surface) and coil or fold it until you have a piece small enough to rest on the bottom of the crucible, but with enough space for air to get in between the coils or folds. It is very important to have as much magnesium ribbon as possible in contact with the bottom of the crucible, as this will be the hottest part of the crucible and we want the magnesium to get really hot!

3. Record the mass of the crucible, cover and magnesium.

4. Setup a ringstand with an iron ring and clay triangle. The height should be adjusted so that when the crucible is resting in the triangle, the hottest part of the flame (the tip of the inner blue cone) is touching the crucible. Strong heating will be needed to make sure all of the magnesium reacts. Tip your crucible slightly one way and put the cover on tipped the other way. The cover should then be about level and is less likely to slide off and break. The reason for the cover is that the reaction produces fine, powdery material that could puff out of the crucible and would not get measured later. However, the crucible cannot be closed to the air because oxygen is needed for the reaction to occur. If we limit the oxygen available, the oxidation will be incomplete and the calculated empirical formula would be incorrect.

5. Upon heating, the magnesium should catch fire and burn with a brilliant white light (powdered magnesium is used in fireworks). You can look for this by keeping an eye on the air gap. The light will be bright enough to see even if you can’t see the magnesium. Heat for a few minutes longer after you see the brightness.

6. Using tongs, remove the cover and continue heating for about five minutes. The bottom of the crucible should have been glowing red for a while. Turn off the burner and let the crucible cool in the clay triangle until the red glow fades away.

7.Then, again using tongs, move the crucible to the countertop and continue cooling until the crucible is near room temperature. If the crucible is too hot, it may be cracked by the addition of water.

8. Add 3 drops of distilled water and heat the crucible again, gently at first, then more strongly, until it has been glowing for about five minutes.

9. Using tongs, set the crucible on the benchtop and let it cool until it has reached room temperature. Record the mass of the crucible, cover and contents.

Analysis and Questions:

1. Determine the empirical formula for an oxide of magnesium, using your data. Perform all the calculations on the data sheet.

2. Why is the distilled water added at the end? Hint: Consider the chemical formula of water and how that might help the outcome of this experiment.

3. Why is the crucible heated after the addition of the water for five minutes?

4. Nitrogen gas (N₂) is a component of air; in fact, it makes up a greater percentage of air than oxygen gas does. Using a similar calculation to that in question 1, show that you probably did not make a nitride of magnesium.

5. Suppose the crucible had not been dry when you weighed it the first time.

a. Would this have led to an “added oxygen” mass that was greater, less than, or the same as the true “added oxygen” mass? Explain your answer.

b. In what way would this have affected your empirical formula? For instance, if the empirical formula for an oxide of magnesium is MgO_x, how would this have affected x? Show how part a of this problem helps you determine this answer.