6: Empirical Formula
- Page ID
- 316201
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Learning Objectives
- To determine the empirical formula of a compound.
An empirical formula of a compound is the simplest whole number ratio of the various atoms in a compound. In this experiment, you will determine the empirical formula of the compound that results when magnesium and oxygen react.
Pre Lab Video
Using a Bunsen Burner
Bunsen burners are a source of heating in the laboratory. However, care must be taken when using a Bunsen burner.
Prior to using:
- Know where all safety equipment is, especially the fire extinguisher.
- Inspect burner for any damage and tubing for any cracks and connect the tubing to gas valve making sure there is a tight fitting connection.
- Remove any papers, notebooks, etc. from immediate working area.
- Tie back and long hair.
- Make sure to be able to get a spark using a striker.
When lighting the burner:
- Turn on gas.
- Use striker to light burner.
- If burner does not ignite within 30 seconds, turn off gas outlet and practice using the striker again.
- Once burner is lit, adjust the gas valve to control the size of the flame.
- Adjust the air inlet to achieve a blue flame with an inner blue cone.
During use of burner:
- Never leave the burner unattended.
- If the flame goes out, turn off gas and wait 30 seconds before reigniting.
- Make sure heated glassware is open and pointed away from any individuals including yourself.
After use:
- Turn off gas as soon as you are finished using it.
- Heated materials will be hot even if they do not look like it. Use extreme caution before touching anything that has be heated.
- Handle heated materials with crucible tongs or heat resistant gloves. This includes ring stands, rings, clay triangles, etc.
- Set hot glassware on hot pads or trivets.
- Allow materials to cool before cleaning up.
Using a Crucible
Crucibles are used to heat solids in the lab. However, handle crucibles with care as they are fragile.
Before using a crucible:
- Check surface for cracks or imperfections.
- Set up ring and clay triangle on a ring stand.
- Place crucible on clay triangle ensuring a good fit.
During use:
- Light burner away from ring and clay triangle.
- Move burner under crucible to heat.
- Heat gently or strongly based on instructions. To heat gently, maintain distance between the flame and the crucible. To heat strongly, adjust burner such that the inner blue cone on the flame is touching the bottom of the crucible.
- Adjust crucible lid for appropriate oxygen levels. Cover tightly to keep oxygen away from sample. Open crucible lid or tilt lid on crucible to allow oxygen to come in contact with sample.
- Use crucible tongs move crucible and lid.
After use:
- Use care when moving the crucible. It will be hot, and it is fragile.
- Place crucible on a hot pad or trivet.
- Do not mass crucible until it is completely cool.
- Be sure gas has been turned off on the Bunsen burner.
Pre-Lab Assignment
In your notebook, explain why sometimes a crucible cover is placed such that it fits tightly on the crucible during heating and other times it is slightly tilted.
Safety and Waste Disposal
- The solids must be disposed of in the hazardous waste container.
Procedure
Safety glasses must be worn at all times.
If the magnesium flashes during heating, do not look directly at the bright light emitted (it could damage your eyes)
Step 1
Clean a crucible and lid, rinsing thoroughly with deionized water as a last step.
Step 2
Place the clean and dry crucible and cover on a clay triangle and heat strongly (flame directly under the crucible and touching the bottom) for 2 minutes to drive off any volatile material.
Step 3
While the crucible is heating, clean thoroughly Mg ribbon weighing approximately 0.3 grams with sandpaper to remove any oxide coating. Avoid handling the ribbon with your finger; this will leave deposits on the Mg ribbon.
Step 4
Using crucible tongs remove the crucible and cover from the clay triangle and place them on a square pad from your drawer to cool.
Step 5
Allow the crucible to cool and weigh the crucible and lid. Handle the crucible with tongs, so you do not leave any deposits from your fingers. Record the weight in your lab notebook.
Step 6
While the first crucible is cooling complete Steps 1 – 3 on a second crucible.
Step 7
Coil the ribbon, without touching it, very loosely and place on the bottom of the crucible. Then, weigh the crucible and lid with the Mg ribbon inside. Record the second weight in your lab notebook.
Step 8
Heat the crucible gently (burner off to the side but nearly touching the crucible) for 2 minutes while using the tongs to lift the cover slightly every 10 sec. to admit air. Should the Mg start glowing brightly when the cover is lifted, cover the crucible, for 10 – 15 seconds. Resume lifting the lid.
Step 9
Heat the covered crucible strongly for 5 minutes continuing to lift the cover occasionally.
Step 10
Lift the cover to determine whether the ribbon has become a whitish ash. If the ribbon is not white ash after 5 minutes, inform your instructor. Heat for a total of 10 minutes ensure the ribbon is no longer reigniting. Allow the crucible to cool. While it is cooling, you should calculate the moles of magnesium you added to the crucible.
Step 11
While the first experiment crucible is cooling complete Steps 7 – 10 with the second crucible and magnesium sample.
Step 12
After the crucible is at room temperature, add 10 drops of distilled water.
Step 13
Partially cover the crucible (leave a slight crack) and heat gently for 1 minute, then strongly for 1 minute. Allow 
the crucible to cool to room temperature. While the first crucible is cooling complete Steps 12 & 13 with the second crucible.
Step 14
Weigh the crucible with product.
Step 15
Reheat strongly (2 minutes), cool and weigh. Repeat this process if the difference between the mass and the mass in number 14 has a difference greater than 0.05 g. *Do not dispose of your sample until your instructor approves your masses.
Step 16
Dispose the product in the waste container labeled "waste magnesium oxide".
Calculations
From the data you have collected, you should be able to find the amount of magnesium in the sample. You should also be able to find the mass of the oxygen that has reacted with the magnesium. From these two masses and the molar masses of magnesium and oxygen, you can calculate the number of moles of magnesium and oxygen present in the final product. Then, you can calculate the empirical formula of the magnesium oxide you made.
Step 1
Calculate the mass of the magnesium added to the crucible.
Step 2
Calculate the mass of oxygen added by finding the difference between the crucible with the final product and the crucible with the magnesium.
Step 3
Calculate the moles of magnesium and oxygen in the product.
Step 4
Determine the empirical formula for magnesium oxide.