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Empirical Formula Determination- Magnesium Oxide Lab

Purpose: To determine the % Composition & Empirical Formula of Magnesium oxide.

Background: We have been talking about the uses of the formulas of compounds as well as how to determine the simplest (empirical) formula of a compound based on chemical analysis. The purpose of this lab is to put this knowledge to use. During this lab you will start with two separate elements and create a compound. Using the mass of the elements that you begin with and the mass of the final product, you should be able to determine the empirical formula of the compound, magnesium oxide.

In this experiment, the percent composition and empirical formula of magnesium oxide, the main compound that is formed when magnesium metal combines with oxygen in air, will be determined. Heating magnesium in the presence of air causes the metal to ignite and burn- lots of light and heat are given off and a new compound is obtained. 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. In the case of the combustion of magnesium, the following equation must be true:

Mass of magnesium + Mass of oxygen = Mass of magnesium oxide

If both the initial mass of magnesium and the final mass of the magnesium oxide are measured, the increase in mass must correspond to the mass of the oxygen that combined with magnesium. The percent composition and empirical formula of magnesium oxide can then be calculated, based on the combining ratios of magnesium and oxygen in the reaction.


• Safety goggles
• Crucible
• Crucible lid

• Crucible tongs
• Clay triangle
• Ring stand & ring clamp
• Bunsen burner
• Wire gauze
• Centigram balance
• Magnesium ribbon, (Mg

Pre-Lab Questions:

A piece of iron weighting 85.65 g was burned in air. The mass of iron oxide produced was 118.37 g.

1. Use the law of conservation of mass to calculate the mass of oxygen that reacted with iron.

2. Use the molar mass of oxygen to calculate the number of moles of oxygen atoms in the product.

3. Use the molar mass of iron to convert the mass of iron used to moles

4. Use the ratio between the number of moles of oxygen atoms to calculate the empirical formula of iron oxide.

5. Calculate the percent composition of iron oxide (what is the %Fe & %O)

6. Draw figure 1 as pictured in your lab notebook

Safety Precautions:

• Magnesium is a flammable metal. Magnesium burns with an intense flame. Do not look directly at burring magnesium. The light contains ultraviolet light that can hurt your eyes. • Do not inhale the smoke produced when magnesium is burned. • Do not lose smoke! The more smoke you lose the more difficult it will be to determine the empirical formula! • Carefully pre-heat the crucible slowly- if you just throw the crucible on the flame- it will cause the crucible to crack- causing you to lose your data and have to restart! • Handle the crucible and its lid only with tongs. Do not touch the crucible with fingers or hands. There is a significant burn hazard associated with handling a crucible. Wash hands thoroughly with soap and water before leaving the lab.

Figure 1

1. Set up the equipment according to figure 1. Do NOT light the Bunsen burner. 2. Adjust the height of the ring clamp so that the bottom of a crucible sitting in the clay triangle is about 1 cm above the burner. This will ensure that the crucible will be in the hottest part of the flame when the Bunsen burner is lit. 3. Using tongs to handle the crucible, measure the mass of a clean, dry empty crucible and its lid to the nearest 0.01g. Record the mass in the data table. 4. Place a coiled 10 cm length of magnesium ribbon in the bottom of the crucible. Measure and record the combined mass of the crucible, lid and...
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