Determining the Empirical Formula of Magnesium Oxide Lab  …

Determining the Empirical Formula of Magnesium Oxide Lab

Overview:

The quantitative stoichiometric relationships governing mass and amount will be studied using the combustion reaction of magnesium metal. Magnesium is reacted with oxygen from the air in a crucible, and the masses before and after the oxidation are measured. The resulting masses are used to calculate the experimental empirical formula of magnesium oxide, which is then compared to the theoretical empirical formula. A crucible and Bunsen burner will be used to heat magnesium metal to burning.

Objectives: 

• Determine the empirical formula and percent yield of the ionic oxide produced by the reaction of Mg with O₂ based on experimental data. 
• Quantitatively and qualitatively evaluate experimental results relative to those theoretically predicted based on known chemical principles and stoichiometric calculations. 

Experimental Notes:

1. Crucibles are used to heat substances to high temperatures (like those encountered with burning metals) without risk of breakage. 

2. Do not touch the crucible with your hands (oils contaminate it and/or you could be severely burned). 

3. Do not place a hot crucible on a lab bench (the temperature difference may cause it to break).

4. Practice using the tongs to pick up the lid from the crucible and the crucible from the clay triangle. 

5. Practice placing the lid partially over the crucible so that there is a gap of about 0.5 cm to let the oxygen gas in.

Procedure:

1. Watch the following video and follow the procedure.

1. Tare a balance with an empty crucible. Do not handle it with your hands. 

1. Obtain about 0.3 g magnesium ribbon and fold the ribbon to fit into the bottom of the crucible. 
2. Record the exact mass of the magnesium ribbon in the data table. 

1. Place the crucible securely on the clay triangle. Set the lid slightly off-center on the crucible to allow air to enter but to prevent the magnesium oxide from escaping. 

1. Place the Bunsen burner under the crucible, light it, and brush the bottom of the crucible with the flame for about 1 minute; then, place the burner under the crucible and heat strongly. 
2. Heat until all the magnesium turns into gray-white powder (probably around 10 minutes). 
3. Stop heating and allow the crucible, lid and contents to cool. Weigh the crucible and its content.
4. Heat the crucible and contents again in order for the crucible to reach to a constant weight. Stop heating the crucible after 5 minutes. Allow the crucible and contents to cool. Weigh the mass of the oxide and record the mass in the data table.

Data:

1. Mass of Mg metal used= 

1. Mass of magnesium oxide formed=

Calculations: Show your calculations for credit. Use proper DA, units, and SF.

1. Write the complete balanced equation for this reaction. Indicate the physical state of each reactant and product.

1. Identify the above reaction.

1. Is this reaction exothermic or endothermic as you perform the experiment? Explain.

1. Determine the mass oxygen used.

1.  Calculate the moles of Mg used.

1. Determine moles of oxygen used.

1.  Use the mole ratio of Mg-to-O, and determine the Empirical formula of magnesium oxide.

1. What is the experimental (measured) percent by mass of O in the magnesium oxide?

1. What is the theoretical (accepted) percent by mass of O in MgO?

1. Calculate the % error of percent oxygen.