Ms. Sonderleiter AP Chemistry Name: Date: Lab #1: Determining the Empirical Formula of a Compound Background: How did early chemists ever manage to determine the chemical formulas of compounds? What kind of experiments did they do? You have learned how to calculate the empirical formulas of compounds using the masses of elements present or the mass percentage of each element, but where do those numbers come from? In this lab, you will need to complete the experiments that experimentally determine those numbers for you to start with and use your values to calculate the empirical formula. You will be reacting a strip of magnesium metal with oxygen in the air to produce magnesium oxide. In order to determine the percent composition and in turn the empirical formula, you will need to record the mass of the sample at several key points during the experiment. The reaction of magnesium with oxygen occurs relatively slowly, so you will be heating the sample with a Bunsen burner to get it to take place more quickly. However, since air is not composed of pure oxygen, the magnesium simultaneously reacts with both oxygen and nitrogen. Thus, in order to find the formula of pure magnesium oxide it is necessary to first convert and magnesium nitride to magnesium oxide. This is done by adding water, which reacts with the magnesium nitride in the mixture to produce magnesium oxide and ammonia gas. Once all of the water and ammonia are evaporated, the crucible should contain pure magnesium oxide. Purpose: To determine the empirical formula of magnesium oxide. Materials: Crucible Crucible cover Bunsen burner Electronic balance Kimwipes Magnesium ribbon Clay triangle Ring stand set-up Tongs Dropper pipet Distilled water
Procedure: 1. Clean a crucible and cover. Dry by heating them in the hottest part of a Bunsen burner flame for 3 minutes. Allow them to cool. Measure and record the mass of the crucible. 2. Cut a 10-cm piece of magnesium ribbon into 1-cm pieces. Try not to touch it with your bare hands, as oils from your skin can affect the mass. Place the pieces in the crucible and measure the mass of the crucible and its contents. 3. Cover the crucible and place it in a clay triangle. Heat gently for 2 minutes. Using tongs, carefully tilt the cover to provide an opening for air to enter the crucible. Heat the partially covered crucible strongly for 10 minutes. 4. Turn off the burner, cover the crucible, and allow the contents to cool. When the crucible is cool enough to touch, remove the cover and examine the contents. If any unreacted magnesium remains, replace the cover at a slight tilt, and reheat strongly for several minutes. 5. Put the cover all the way on and allow to cool. After making sure that all the magnesium has reacted, use a dropper pipet to add enough distilled water to the crucible to just cover the contents. Wash any material that may have spattered onto the inside of the cover into the crucible. 6. Holding the burner in your hand, gently heat the contents of the uncovered crucible by moving the burner slowly back and forth. Avoid spattering. Observe the odor of the vapor given off. 7. When all of the liquid has boiled off, repeat steps 5 and 6. 8. When all the liquid has boil off a second time, strongly heat the uncovered crucible for 5 minutes. 9. Turn the burner off and allow the crucible and contents to cool. Measure the combined mass of the crucible and contents. Data: Data Mass (g) Mass of empty crucible (dried) Mass of crucible and magnesium Mass of crucible and magnesium oxide Observations:
Calculations: 1. Calculate the mass of magnesium used: 2. Find the mass of oxygen that reacted: 3. Determine the empirical formula of the magnesium oxide compound: Conclusion Questions: 1. How do your results for the empirical formula compare to the actual empirical formula of magnesium oxide (MgO)? Explain.
2. Justify any deviation from the theoretical results. (For example, if your ratio was too high or low, cite and explain specific reasons why this might be the case.) 3. If you did not heat the magnesium long enough for full reaction of the metal, how would your empirical formula differ? Explain. 4. What is the percentage of magnesium by mass in magnesium oxide?
5. Suppose 2.033 g of magnesium is reacted according to an experiment similar to the one above. What is the theoretical yield of magnesium oxide? 6. A sample of sulfur having a mass of 1.28 g combines with oxygen to form a compound with a mass of 3.20 g. What is the empirical formula of the compound?