Determining the Empirical Formula of a Compound

Honors Chemistry Lab

Objectives

The objectives of this lab are

  1. Follow lab procedure for the use of crucibles.
  2. Determine the empirical formula of a compound.

Background

In this experiment you will determine the empirical formula of magnesium oxide. Magnesium oxide forms when magnesium metal is burned in air, combining with oxygen. According to the law of conservation of mass, the total mass of the products of a chemical reaction must equal the total mass of the reactants.

mass magnesium + mass oxygen = mass magnesium oxide

From the mass of magnesium used and mass of magnesium oxide formed, you will find the mass of oxygen used in the reaction. The masses of magnesium and oxygen used will be converted to moles. The ratio of the number of moles of magnesium to the number of moles of oxygen will be calculated. This ratio will be converted to a simple whole number ratio. From this ratio, the empirical formula can be written.

Safety

1.  Wear safety goggles.

2.  Handle the crucible only with the tongs. There is a significant burn hazard associated with the handling of crucibles because a hot crucible looks exactly like a cold crucible.

3.  Remove the gas burner from beneath the crucible before using the crucible tongs to remove the crucible and its lid. Use the tongs to grasp the lid by its porcelain knob; the crucible should be grasped by one of its edges. (See the figure 1 below.)

Figure 1 Figure 2

4.  If you must adjust the height of the ring support, remove the crucible lid before doing so. Otherwise, it may fall when you move the ring.

5.  Do not look directly at burning magnesium. The intense light given off in this reaction may hurt your eyes.

6.  Do not inhale the "smoke" produced when the magnesium is burned. Keep your face at arm's length from the crucible.

7.  Do not stir or otherwise disturb the product in the crucible. Dump the entire contents of the crucible into the sink and rinse the sink with water.

Materials

Safety goggles, crucible with crucible lid, crucible tongs, clay triangle, ring stand, ring support, balance, gas burner. magnesium (Mg) ribbon, scissors, ruler

Procedure

1.  Place a crucible on a clay triangle balanced on a ring support clamped to a ring stand. Light the gas burner and adjust it to give a clear blue flame. (A yellow flame will deposit soot on the crucible. Why would this be a problem?) Place the burner under the crucible. Adjust the height of the ring support so that the bottom of the crucible is in the hottest part of the flame. Place the crucible lid slightly ajar on the crucible. (The crucible lid should be large enough to fit loosely down over the crucible edge.) (See Figure 2 above.)

2.  Heat the crucible so that its bottom glows red for five minutes. Remove the burner and allow the crucible and crucible lid to cool. This will take at least 10 minutes. CAUTION: The crucible gets extremely hot. Never touch it. Always use crucible tongs in handling this piece of equipment. When the crucible and lid are completely cool, use crucible tongs to transfer them into a beaker and carry it to the balance. Do not place a hot crucible on the balance. Inaccurate mass readings and damage to the balance may result. Determine the mass of the empty crucible and lid to the nearest 0.01 g. Record this mass.

3.  Obtain about 25 centimeters of magnesium ribbon. Curl the ribbon around a pen or pencil and place it in the bottom of the crucible. Determine the combined mass of the crucible, crucible lid, and the magnesium. By placing it on the balance. Record this mass in the data table.

4.  CAUTION: Do not look directly at the burning magnesium. The intense light may injure your eyes. Place the crucible, and lid, on the clay triangle. Heat the crucible strongly until the magnesium ignites. CAUTION: Be careful to keep the crucible at arm's length at all times. Do not inhale the "smoke" produced. After the reaction has subsided and "smoke" production has ceased, heat the crucible for at least 5 minutes CAUTION: Do not lean over the crucible. After heating, remove the burner and allow the crucible to cool completely (at least 10 minutes).

5.  When the reaction is completed, the magnesium should be wholly converted to a light gray powder. If magnesium ribbon like material remains in the crucible, heat the crucible and its contents for another 10 minutes. Allow the crucible to cool completely (at least 10 minutes) before another massing to determine the combined mss of the crucible, crucible lid, and magnesium oxide. Record your results. The product can be thrown in the trash. Wash your crucible and return it to the container.

Name ______Block ______

Data

Empty Crucible and Lid After Heating (g)
Crucible, Lid and Magnesium (before heating) (g)
Crucible, Lid and Combustion Product MgxOy (g)

Calculations

1.  Calculate the mass of magnesium used.

2.  Calculate the moles of magnesium used.

3.  Calculate the number of atoms of magnesium reacted.

4.  Calculate the mass of the combustion product.

5.  Calculate the mass of oxygen that combined with the magnesium.

6.  Calculate the moles of oxygen in the combustion product.

7.  Calculate the moles of oxygen molecules that reacted with the magnesium.

8.  Express the ratio between the number of moles of magnesium and oxygen in the combustion product in the smallest whole number ratio.

9.  Use the smallest whole number ratio to write the empirical formula for magnesium oxide.

Questions

1.  How is an empirical formula different than a molecular formula?

2.  Explain why having more mass in your crucible when the reaction was over does not violate the law of conservation of mass.

3.  Explain how you could determine the empirical formula of the compound by using the periodic table.

4.  Write the balanced equation for the reaction of Magnesium and Oxygen.

5.  How do your answers to calculations #2 and #7 compare to balanced equation?

6.  Discuss reasons why your compound is ionic or covalent.

Discussion of Error

This lab does have experimental error. Try to think about what could cause your calculations to be high or to low. We will discuss this in class. But, you should be considering it as you go through the lab. Also compare your results to the actual empirical formula that will be given in class. No percent error calculation is necessary.

Be sure to include the “Discussion of Theory” and “Conclusion” in your report.

Reference

Adapted from

EMPIRICAL FORMULA DETERMINATION. Morrow High School. 12 Mar.
2005
<http://www.clayton.k12.ga.us/schools/003/labs/chemistry/EmpiricalF
ormula00.pdf>.