Purpose

The purpose of this lab is to use physical methods to determine the chemical formula of an oxide.  Oxides can be formed when a substance burns, but they are often gases and therefore hard to collect.  We will be investigating a compound that make a solid oxide upon combustion.

Procedure

1. Record all data necessary to determine the initial mass of a small piece of magnesium ribbon (or pile of magnesium turnings).
2. In a small crucible, heat the magnesium with a Bunsen burner until it has completely burned.
3. After the product has cooled, add a small amount of water to decompose nitride impurities.  Let the mixture sit for a few moments, then gently heat it to dryness.
4. Record the necessary data to determine the substance's final mass, and include observations.

Calculations

1. Initial mass of magnesium.
2. Final mass of magnesium oxide.
3. Mass of oxygen that reacted with the magnesium.
4. Moles of magnesium present.
5. Moles of oxygen present.
6. Simplest whole number ratio of moles of magnesium to moles of oxygen. Write this as a chemical formula (for example, H2O).

Questions

1. Does the magnesium in your crucible gain or lose mass?  Explain.
2. How would your final ratio change if not all of the magnesium reacted?
3. How would your final ratio change if your product was still moist at the end?
4. What formula would you predict for magnesium oxide, based on the elements' positions on the periodic table?
5. Does your calculated formula for magnesium oxide (calculations #6) match the predicted formula (questions #4)?

Write Up

The usual format.  Make sure you include everything listed above, in order and numbered.  Talk to your partner to make sure your data and calculations agree, and make sure your reports are complete and correct before you pass them in. The entire report must be on only one sheet of paper.

[Formula of Magnesium Oxide score sheet][MHS Chem page]