Determination of the Empirical Formula for a Compound*
(Adapted from Experiment 4C – Heath Chemistry)
Introduction: In this experiment, a measured mass of magnesium metal will be reacted with an excess of 2 M HCl. The
excess hydrochloric acid and water will be evaporated, and the mass of the magnesium chloride product will be measured.
This data will be used to calculate the empirical formula for magnesium chloride.
Objectives:
1. to determine the mass of magnesium chloride produced when a given mass of magnesium is reacted with an
excess of 2M HCl.
2. to calculate the simplest formula for the compound magnesium chloride.
3. to compare the simplest formula obtained experimentally with the accepted formula
4. to analyze the cause of any unexpected experimental results
Hypothesis: In this experiment, you will be experimentally determining the formula for a compound of magnesium and
chlorine. Write a hypothesis with the expected formula for the compound. Be sure to explain your reasoning.
Materials
centigram balance
beaker tongs
150 ml beaker
magnesium metal turnings
25 ml graduated cylinder
10 – 12 mL 2 M HCl
hot plate
wire screen
Procedure:
1. Read through the procedure. Design a data table for the data you will collect.
2. Put on goggles !!!
3. Label and then measure the mass of a clean, dry 150 mL beaker. *Record this mass.
4. Measure approximately 0.11 - .14 g of magnesium into a small cup. * Record this mass.
5. Place the magnesium in the beaker. Slowly add 8 ml of 2M HCl to the beaker. (If the solution bubbles over,
you will receive a 0 on the lab.) Observe the reaction. *Record your observations.
6. Continue to slowly add HCl until the reaction stops.
7. Place your beaker on the hot plate, set the hot plate at 5 or 6.
8. Your goal is to slowly dry the solution. This will take about 10-15 minutes. When the solution is almost dry, turn the
hot plate off and let the compound continue to dry. [The hot plate stays hot for several minutes.]
9. Use the beaker tongs to remove the beaker from the hot plate. Set it on the wire screen.
10. Allow the beaker to cool for 4 or 5 minutes and mass the beaker and its contents. *Record this mass.
11. *Record your observations of the beaker’s contents.
12. Turn the hot plate to 4 or 5 and heat the beaker for 2 more minutes, then allow it to cool for 2 minutes.
13. Find the mass of the beaker and its contents.
14. If the mass has gone down, repeat steps 12 and 13. Continue to repeat these steps until the mass remains constant.
Be sure to indicate the FINAL, LOWEST mass on your data table.
15. Determine the mass of the magnesium chloride. *Be sure this value is shown on your data table.
16. Wash the magnesium chloride down the drain, clean up your lab area, and wash your hands.
Analysis: Answer these questions using complete sentences or labeled equations.
1. State the law of conservation of mass.
2. Determine the mass of chlorine in the compound produced. Show labeled steps for your work. Assume all
of the original magnesium was used in the reaction.
3. Determine the number of moles for both magnesium and chlorine in the new compound.
4. Calculate the ratio of the number of moles of magnesium and chlorine. Write the experimentally
determined empirical formula.
Conclusion: Write a short conclusion comparing the results of your experiment and the predicted formula.
Evaluation: Explain, in some detail, two factors that seem to have affected your results. You may NOT use the words
“human error”.
PLEASE do NOT write on this paper. Return the paper at the end of class.