Lesson 4: Lab Work
In the lab this week you will combine a number of the skills you have learned to measure and compute the percent composition of magnesium oxide. Before you go to the lab, let’s review the procedure you’ll use.
Specifically, the skills you have learned that you will use in this lab are:
- how to determine the mass of a sample of material on a precision electronic balance;
- how to light and adjust a Bunsen burner;
- how to record data a perform calculations to the correct number of significant digits;
- how to compute the percent composition of a compound from mass data;
Basically, the idea is to react a known mass of the magnesium metal with the oxygen in the air, and then determine the mass of magnesium oxide that forms. The procedure is designed to make sue that all of the magnesium forms magnesium oxide, and that none of the magnesium oxide is lost.
You will induce the reaction between magnesium and oxygen by heating the magnesium to high temperature. Under these conditions, the magnesium also reacts with the nitrogen in the air, so you will have to convert the compound of magnesium and nitrogen that forms to magnesium oxide.
The reaction can be violent, so you will also take steps to minimize the spattering that might occur and contain what spattering you can’t prevent.
First, dry a crucible and its cover by heating them gently over a Bunsen burner. Let them cool to room temperature, then weigh them on a precision (electronic) balance.
Heat the crucible gently by increasing the distance between the flame and the bottom of the crucible – don’t use a cool, smoky flame or you will get soot all over the bottom of the crucible.
Also, be sure the crucible has no cracks. Heating a cracked crucible, even if the crack is small, can cause it to break, and then you would have to start the experiment over from the beginning.
Next, cut a strip of magnesium ribbon about 20 – 50 centimeters long, clean it with steel wool, then weigh it on the electronic balance.
It is better to make the magnesium ribbon a bit too long than a bit too short.
Clean the ribbon with steel wool until it is shiny. Dark coloration on the ribbon is magnesium oxide and we need to know the mass of the magnesium only.
If you use a corroded piece of ribbon, the mass of the magnesium you measure on the balance will include some magnesium oxide. Your value for the mass of the magnesium will therefore be too high and your value for the mass of the magnesium oxide will be too low.
Now coil the ribbon loosely or crumple it so air can circulate around it and place it in the crucible. Close the cover, then heat the crucible, first gently, then until it gets red hot, over a Bunsen burner. It should stay red hot for about 15 minutes.
It is important not to pack the ribbon too tight. Doing so will prevent oxygen from reaching all of the magnesium, so the value you get for the amount of magnesium oxide that forms will be too low.
Hearing the magnesium gently at first prevents the reaction from occurring too violently. If the reaction is too violent, some of the magnesium oxide is ejected from the surface of the magnesiuim as a fine, smoky powder and can excape from the crucible.
Covering the crucible helps keep inside any magnesium oxide powder that is ejected.
Use tongs to open the cover a little, and continue heating for another ten minutes. At the end of the time let it cool for a few minutes, then have the instructor check to make sure all the magnesium has reacted.
Opening the cover admits more oxygen. By this time, there is little danger of spattering and what little magnesium has not reacted is coated with magnesium oxide, which tends to keep oxygen away from the unreacted metal. Admitting more oxygen helps guarantee that all of the magnesium will react.
If the magnesium has all reacted, the material in the crucible will be a fine greyish-white powder that crumbles easily when touched with the end of a small spatula. If the ribbon of ash tends to maintain its shape when prodded, it is likely there is unreacted magnesium holding it together.
The crucible now contains a mixture of magnesium oxide and magnesium nitride, the product of the reaction of magnesium with the nitrogen in the air.
Magnesium nitride reacts with water to form magnesium oxide, releasing ammonia in the process.
Using the pipette or dropper, add about 15 drips of distilled water, replace the cover, and heat gently for 5 minutes. At the end of this time, allow the crucible to cool to room temperature, then weigh it on the electronic balance.
If you are careful you may be able to detect the odor of ammonia when you add the water. It will probably be faint, but sniff gently and be careful not to burn your nose on the hot crucible!
Your data consist of the mass of the crucible and cover, the mass of the magnesium, and the mass of the crucible and cover the magnesium oxide together. You can subtract the mass of the crucible and the cover from that of the crucible, cover and magnesium oxide to get the mass of just the magnesium oxide.
As you perform the calculations for this lab, pay close attention to the number of significant figures. You may have as many as five significant digits in the mass of the crucible, cover, and magnesium oxide, and as few as three significant digits in the masses of the magnesium and the magnesium oxide.
You are performing both addition/subtraction and multiplication/division in the course of your calculations. Remember that significant digits are handled differently in the two cases. Be sure you examine each step of your calculations to make sure you know how many significant digits should be used in the next step.
You can then determine the mass of the oxygen that must have reacted with the magnesium by subtracting the mass of the magnesium from that of the magnesium oxide.
Once you know the mass of the magnesium, the mass of the oxygen, and the mass of the magnesium oxide, it should be a simple matter to calculate the mass ratio of magnesium to oxygen and the percent magnesium and percent oxygen in magnesium oxide.
Remember that the percents of the magnesium and oxygen must add to 100% but that small deviations from this are probably due to rounding errors and can usually be ignored.
Example 9 in your workbook reviews the lab report format. Use this as a model to prepare your own lab report on a separate sheet of paper. Be sure to include all the sections you see listed.