This pair of experiments uses gravimetric principles to determine basic chemical data. The first experiment finds the molar mass of magnesium, the second finds the formula of a hydrate. Both experiments are written in a format that can be fitted easily to worksheets. They are written at a low level, providing all steps for calculations. Feel free to modify them to utilize more critical thinking skills. With more advanced students, they should be able to "invent" this procedure themselves, if you give them enough background to work with.

Massing Magnesium Metal

PURPOSE - to experimentally determine the molar mass of magnesium

MATERIALS - Goggles, Bunsen burner, matches, ring stand (with ring), clay triangle, crucible with lid, balance, crucible tongs, magnesium strip, desiccator (and desiccant)

REACTION - 2 Mg + O2

PROCEDURE - Wear your goggles at all times. Hot gasses and dust are formed, and can burn or scratch your eyes. Use crucible tongs to handle hot objects. Do not handle any objects with your fingers, as they add oils (and mass) to anything they touch.

1. Set up your ring stand, clay triangle and burner so that the crucible will rest above the hot part of the flame.

2. Heat the empty crucible with the lid on it for five minutes to drive off any water.

3. Using tongs, place the HOT! crucible and lid in the desiccator to cool.

Once cool, weigh and record the mass of the empty crucible and lid.

4. Place a magnesium strip into the crucible. (Handle with tongs or gloves, or paper towel - but not fingers!) Record the mass of the crucible, lid and magnesium combined.

5. Heat the covered crucible and magnesium over the flame. After 15 minutes, use the tongs to raise the lid slightly, letting some fresh oxygen in. Repeat after another 5 minutes, and once more after another 5 minute period. Be careful not to raise the lid much, or some of the MgO will escape as a white smoke.

6. Extinguish the flame and return the HOT! crucible to the desiccator to cool.

Once cool, weigh and record the mass of the crucible, lid and magnesium oxide.

7. Look inside the crucible to observe the product. Compare its appearance to the original magnesium. If any of the magnesium strip appears to be unreacted, repeat the procedure starting from step 5.

DATA - Record your masses and observations.

Unreacted Mg :

Product MgO:

A) Mass of Crucible + Lid: ____________________g

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