|Magnesium reacting with oxygen in a red-hot crucible|
Photo by Julia Paneyko
The procedure is very simple: heat magnesium in a crucible until it is fully reacted with oxygen. Use the mass data of the magnesium before reaction and product after reaction to calculate the mole ratio of Mg and O in the compound. This mole ratio is the experimental chemical formula of magnesium oxide. My "go to" procedure for this lab is found in ChemTopic Labs Volume 7: Molar Relationships & Stoichiometry by Flinn. In Flinn's version of the lab, there are detailed instructions to heat the magnesium with the lid on, then open the lid every three minutes, and then heat it some more with the lid at a tilt. All of this manipulation of the crucible lid invariably results in at least one dropped and broken lid each lab period. And, the results of the lab are usually not that great, with many groups getting ratios other than the expected 1:1.
|My students are heating their crucibles with lids off.|
Here's where the big payoff was obvious. The lab results were much better this year, with most groups getting the expected 1:1 ratio without the need for sad stories about experimental errors in the conclusion. Taking the lid off resulted in better results, more interesting observations during the reaction, and better results for the chemical formula for magnesium oxide.