The empirical formula of a compound is the formula which shows the simplest whole number ratio between atoms in a compound. For example, glucose , C₆H₁₂O₆ , would have an empirical formula of CH₂O , since the 6:12:6 ratio of carbon:hydrogen:oxygen can be reduced to a simplest whole number ratio of 1:2:1.

In this lab we will determine the empirical formula for magnesium chloride. We will do so by starting with a given amount of magnesium and reacting it with hydrochloric acid to make magnesium chloride. Once we weigh the dry product, we can subtract the mass of magnesium which we used to find out how much chlorine attached itself to the magnesium. This will allow us to determine the ratio of moles of magnesium to moles of chlorine in the magnesium chloride, which will give us a simple whole-number ratio empirical formula for the compound.

Prelab Questions. (answer in your lab book.)

1. Given your knowledge of how to write chemical formulas, write the formula for magnesium chloride.

2. In this reaction we react solid magnesium with aqueous hydrochloric acid (HCl) to form a solution of magnesium chloride and a gaseous product. Predict what that gas will be.

3. Write a balanced chemical equation for the reaction described in question # 2.

Procedure. (Safety Goggles are required!)

Day 1

1. Wash and dry a 150 mL beaker.     Write your initials on it with a pencil.  Weigh and record the weight of the clean, dry beaker.

2. Add approximately one gram of magnesium to the beaker.  Record the weight of the beaker + magnesium.

3. Add 1M HCl to the 100 mL mark in the beaker.  Record your observations. If any Mg remains after ~15 minutes, add another 30 mL of HCl to the beaker.

4. When all the Mg is reacted, put the beaker on the hot plate in the fume hood.  We'll evaporate water from the solution overnight, leaving you with a dry product of magnesium chloride.

Day 2

1. Remove your beaker from the drying oven. CAUTION! HOT! USE BEAKER TONGS!  Set the beaker on the counter top until it is cool to the touch.  Record your observations of the beaker contents.

2. Measure the mass of the beaker and magnesium chloride. Record the data.

3. Scrape the product from the beaker into the trash can, using a spatula for a scraper.  Wash the beaker with soap and water and hang it to dry on the drying rack.

4. Do the lab calculations.

Calculations and Questions.

1. Calculate the mass of magnesium which reacted.

2. Calculate the mass of magnesium chloride which was produced.

3. Calculate the mass of chlorine which was present in the magnesium chloride.

4. Calculate the number of moles of magnesium in the compound.

5. Calculate the number of moles of chlorine in the compound.

6. Calculate the ratio of moles of magnesium to chlorine .

7. Use this ratio to determine your experimental value for the empirical formula of magnesium chloride.

8. Compare this value to your answer to Prelab Question #1. Discuss any discrepancies.

9. Write a conclusion to the experiment. Include a discussion of experimental errors which are specific to the results which you obtained.