The Magnesium Oxide Lab
In a compound the atoms of different elements are present in numbers whose ratio is usually an integer or a simple fraction. The simplest (or empirical) formula of the compound expresses that ratio.
Simplest formulas are determined by establishing the mass of each element present in a sample of the compound. From those masses one finds the number of moles of each element present. The mole ratio is also the atom ratio in the compound and that ratio provides the subscripts for the simple formula.
To find the mass of each element in a compound one must carry out at least one chemical reaction. Sometimes it is possible to form the compound directly from its elements. This is called a synthesis. In this experiment we will synthesize magnesium oxide by heating magnesium in the oxygen in the air:
magnesium + oxygen magnesium oxide
By weighing the magnesium before the reaction occurs, and the magnesium oxide produced after, one can calculate the mass of the oxygen that reacted with the magnesium. To obtain good results in this experiment you must make each weighing precisely.
1. To determine the simplest formula of magnesium oxide.
2. To find the percent magnesium in magnesium oxide.
Place a clean crucible and cover on a clay triangle on an iron ring (Fig. 5- 2). The crucible cover should be tilted leaving a small opening. Heat the crucible strongly for about 1 minute to drive off any moisture. Allow the crucible and cover to cool to the touch and then weigh them together. 2.
Obtain a piece of magnesium ribbon approximately 50 cm long. Coil the magnesium and add it to the crucible. Weigh the crucible, cover and magnesium. 3.
With the cover off, heat the crucible. Increase the temperature gradually. When the magnesium ribbon glows red, or ignites, cover the crucible quickly and reduce the amount of heat applied. To prevent any loss of product, the crucible must be covered when you observe ignition. After about a...
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