Percent Composition and Empirical Formula Lab Evaluation

Note you will not be completing this lab sample results will be provided for your use.

Introduction Magnesium burns in the presence of oxygen to produce magnesium oxide. In this experiment you will determine the percentage composition of magnesium oxide and calculate its empirical formula.

Purpose To determine the percentage composition and empirical formula of magnesium oxide.

Hypothesis Predict the empirical formula for magnesium oxide.

Material

Solids

magnesium ribbon

Apparatus

crucible and cover, Bunsen burner, support stand, clay triangle, electronic balance, wire gauze, crucible tongs, sandpaper

Safety

Safety goggles and lab aprons should always be worn when working with chemicals.

This lab will be performed using small quantities of chemicals (micro-scale chemistry).

Burning magnesium produces a very bright white flame. Do not look directly at the flame; it can damage your eyes.

Be careful when handling hot items in the lab.

Procedure

Use a piece of sandpaper to clean a 16 cm piece of magnesium ribbon. This will remove any oxide coating and impurities on its surface.

Clean and dry the crucible and lid.

Determine the mass of the empty crucible, and lid using an electronic balance, precise to 0.01g

Roll the magnesium into a loose coil in order for it to fit inside the crucible.

Add the magnesium to the crucible.

Determine the mass of the magnesium, crucible, and lid using an electronic balance, precise to 0.01g

Place the crucible on the clay triangle on the support stand, about 5-10 cm above the Bunsen burner.

Cover the crucible with the lid and heat strongly for 5 minutes.

Using crucible tongs, lift the lid slightly to allow air to enter into the crucible and to briefly observe the reaction process.

Once the magnesium no longer glows and the reaction has stopped, remove the crucible and let it cool.

Determine the mass of the crucible, lid and product.

Clean your material and dispose of the waste in the correctly labelled waste container.

Sample Results Table #1: Images of reactants that you would have worked with if you had done the actual experiment.

Chemical	Image of substance before the reaction
Mg

Table #2: Images of what you would have obtained in the lab if you had done the actual experiment.

Chemical	Image of product
Mg

ecord your observations for each reaction in a table like the one below.

Item	Mass (g)
Crucible & lid	23.18
Crucible, lid & magnesium	25.97
Magnesium
Crucible, lid & magnesium oxide	27.69
Magnesium oxide

Calculations

Complete the results table.

Determine the percentage composition of your product.

Using the percentage composition, calculate the empirical formula for your magnesium oxide product.

Conclusion Briefly state the results for the laboratory, in general terms. This should be no more than 2 sentences.

Questions

1) Explain why moles must be used to calculate the empirical formula, and not the mass.

2) Suppose some magnesium oxide had escaped during the heating. How would the empirical formula have changed?

3) Explain how the empirical formula for magnesium oxide would have changed if all of the magnesium in the crucible had not burned.

4) State 3 sources of error for this experiment. These should be conditions that may have changed to influence the results.