Determining the Empirical Formula of Magnesium Oxide

Determining the Empirical Formula of Magnesium Oxide
Background
The empirical formula of a compound describes the composition of a compound in terms of the
simplest, whole-number ratio of atoms in a molecule or formula unit of the compound. In this experiment,
the empirical formula of magnesium oxide will be determined. When magnesium is heated in the presence of
oxygen, the metal ignites and burns. According to the law of conservation of mass, the total mass of the
products of a chemical reaction must equal the mass of the reactants.
Mass of magnesium + Mass of oxygen = mass of magnesium oxide
If both the initial mass of magnesium and the final mass of magnesium oxide are measured, the
increase in mass must correspond to the mass of oxygen that combined with the magnesium. The mole ratio
of the two can then be calculated to determine the empirical formula of the compound.
Materials
Magnesium ribbon, ceramic crucible/cover, bunsen burner, ring stand, clay triangle, crucible tongs,
electronic balance (+ 0.01g)
Procedure
1. Determine the mass of a clean, dry crucible.
2. Obtain a 25-cm piece of magnesium ribbon. Polish with steel wool until shiny. Coil the ribbon around
a pencil to form a loose coil. Determine the mass of the ribbon.
3. Heat the covered crucible gently for 2-3 minutes.
4. Heat the crucible with the hottest part of the flame for 3 minutes. After 3 minutes, lift the lid a small
amount. Replace the lid. and heat for another 3 minutes. Lift the lid a small amount. Continue heating
and lifting the lid every 3 minutes for a total of 15 minutes. You will have lifted the lid a total of 5
times.
5. After 15 minutes, turn off the gas and allow the crucible to cool.
6. Determine the mass of the crucible.
Data
1. Mass of crucible
2. Mass of crucible + Mg ribbon
3. Mass of Magnesium (#2-#1)
4. Mass of crucible + product (after heating)
5. Mass of Magnesium Oxide (#4-#1)
6. Mass of Oxygen (#5-#3)
Analysis
1. Use the molar masses of magnesium and oxygen to calculate the moles of each reactant.
2.
Calculate the ratio between the number of moles of magnesium and the number of moles of oxygen.
What is the empirical formula of magnesium oxide?
3.
Calculate the percent composition of magnesium oxide.
Questions
A piece of iron weighing 85.65 g was burned in the air. The mass of the iron oxide produced was 118.37 g.
1. What mass of oxygen reacted with the iron?
2. Calculate the number of moles of iron that reacted.
3. Calculate the number of moles of oxygen that reacted.
4. Use the ratio between the moles of iron and oxygen to determine the empirical formula of iron
oxide. Note: Fractions of atoms do not exist. In the case where the ratio of atoms results in a
decimal fraction, such as 1.5:1, the ratio should be simplified by converting it to the nearest whole
number ratio. The ratio 1.5:1, for example, is converted to 3:2 by multiplying both terms by two.

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