Question

Procedure:

First, weigh out about 0.5 grams of magnesium metal, as shavings or ribbon.

Always be sure to note the exact mass in your lab notebook, to as many decimal places as the balance displays. This will determine the number of significant figures in your

calculations. The mass of the magnesium ribbon was 0.51 g.

Next, obtain a crucible. Don’t wash the crucible in water; ceramic material acts like a sponge and will soak up lots of water, which will cause all kinds of problems in

this experiment. Get the mass of the crucible and write it down your lab notebook.

2.72 g

Set up a ringstand with a clay triangle. Place the crucible with your magnesium sample into the clay triangle. Heat the crucible and magnesium thoroughly with a Fisher burner flame. Oxygen gas from the atmosphere will react with the magnesium. Don’t look directly at the burning magnesium! You will notice the magnesium become a white, fluffy mass as it reacts to become magnesium oxide. Heat the crucible at high heat for at least ten minutes, and then allow the crucible to cool to room temperature. Transport the crucible with tongs from here onwards, otherwise the sweat and oils from your skin could add extra mass. Weigh the crucible containing the magnesium oxide and note this mass in your lab notebook. Notice that the mass has increased due to the addition of oxygen

and nitrogen to the magnesium. The mass of the crucible filled with ash is 3.55 g

To remove any magnesium nitride that may have formed, add about five to ten drops of distilled water to the ash, enough to get all of the ash slightly damp. Again heat the crucible on the clay triangle for about ten minutes. After a minute or so of heating, cautiously sniff as demonstrated by your instructor, you should be able to smell a faint odor of ammonia. This indicates that some magnesium nitride was produced and is now being decomposed. After the ten minutes of heating, the ash should appear completely dry, if not, heat some more. Allow the crucible to cool to room temperature and find the mass of the crucible and ash. Once again, heat the crucible and ash for another ten minutes and allow to cool to room temperature. Weigh a second time. If the second mass closely matches the previous mass, you can proceed. Otherwise, repeat the heating and cooling process again. By heating the crucible and ash, you are driving away any water that may have been absorbed into the ceramic material or the ash. When continued heating doesn’t decrease the mass any more, you can safely say there is no more water contained within the ceramic material or the ash. This is called “Heating to Constant

Mass”.

Final mass of crucible filled with ash is 3.57 g

The crucible is disposable and can be thrown away when you are done with the

experiment.

Data Analysis

Mass of magnesium                                                                                        grams Final mass of magnesium oxide                                                                                       grams

Mass of oxygen                                                                                               grams ( mass of magnesium oxide – mass of magnesium)

Take the mass of magnesium and divide by 24.3050 and note the answer as moles of magnesium.

Take the mass of oxygen and divide by 15.9994 and note the answer as moles of oxygen. (Where did these numbers come from?)

Divide moles of oxygen by moles of magnesium.

The answer is the ratio of oxygen atoms to magnesium atoms.

Round this answer to the nearest whole number and use it to determine the empirical formula of the magnesium oxide you made. In this case, the empirical formula will be the same as the molecular formula. Is this the same as the formula you would have predicted for magnesium oxide?

(For example, if we were determining the formula of sulfur trioxide, and the ratio of oxygen atoms to sulfur atoms was approximately 3, we would write the empirical formula of sulfur trioxide as SO₃. This indicates there are three oxygen atoms for every one sulfur atom.)

Questions:

1. If your answer required minimal rounding to get to a whole number, that indicates a high level of accuracy in your results. If a lot of rounding was necessary, that indicates a low level of accuracy. What was the accuracy of your results? If your accuracy was low, what sources of error might have contributed to the lack of accuracy?

2. If a magnesium fire got out of control, would it be advisable to try to put it out with water? What about a carbon dioxide fire extinguisher?

Answer 1