Empirical Formula Pre Lab Answers

A) January 18, 2011B) Empirical Formula
C) The purpose is to determine the empirical formula of a metallic oxide.
D) Pre Lab Questions:
After heating the metal, the crucible and contents should mass less than it did before it was heated. This is because heating the crucible may rid of other residue that was left in it; bringing it a to a constant mass.
A yellow flame will deposit soot on the crucible. This would be a problem because the soot left on the crucible would vary from our constant mass and cause our mass to be inaccurate.
We have to heat the crucible first without any material in it because we want to have the crucible heated at a constant weight.
The chemical make-up of the compound being produced will be magnesium oxide.
You must carefully
sand the outside of the magnesium ribbon to remove any oxide coating.

E) Apparatus:
Crucible
Tongs
Mass
Bunsen Burner
Magnesium Ribbon
Clay Triangle
Ring support clamped to a ring stand

F) Safety Precautions/ Waste Disposal:
Wear goggles and aprons at all times.
Tie back hair and roll up sleeves.
Handle crucible with proper tongs.
Do not look directly at the burning metal!
Do not stir or otherwise disturb the product in the crucible.
When finished, discard the entire contents of the crucible into a designated beaker provided by the instructor.
Keep hot equipment at safe distances and do not touch till it is has cooled for the instructed amount of time.

G) Procedure/Observations:

Procedure Observation Conclusion
Place a crucible on a clay triangle balanced on a ring support clamped to a ring stand.
Note: Don’t use the lid to the crucible.
Place a lighted burner under the crucible and adjust the ring support so that the crucible is in the hottest part of the two cone blue flame.
Note: It is important to get the tip of the inner cone directed at the bottom of the crucible. The hottest part of the flame is the inner cone of the flame.
Heat the crucible so that the bottom glows red for two minutes.
Remove the burner and let the crucible cool for ten minutes.
While the crucible is cooling, obtain a piece of magnesium ribbon. Sand it twice and obtain mass.
Note: Magnesium Ribbon is pure magnesium.
After the crucible is cooled, remove it with tongs and obtain
mass.
Note: Use the same balance the whole time throughout the lab.
Coil the sanded ribbon so it lays flat on the bottom of the crucible.
Place the crucible with coiled magnesium ribbon inside on the clay triangle.
Heat strongly until metal ignites. Once the metal ignites, pull the burner away.
Note: Do not look directly at the burning metal.
After the reaction has subsided, heat the crucible for another five minutes. Allow crucible to cool completely.
Mass the crucible and its contents.
Dispose of the contents in a designated beaker.

H) Data:

Mass of Objects
Object Mass (g)
Empty crucible (after heating)
Sanded Magnesium Ribbon
Crucible and Magnesium Ribbon (after heating)

I) Calculations:

Determine the empirical formula of the compound synthesized. Do not try to make the subscripts whole numbers; round to the nearest hundredth; after getting one element to equal one.

J) Final Conclusion:

K) Post Lab Questions:

1. Using your data, calculate the percentage by mass of oxygen in your compound.
2. Calculate the theoretical percentage by mass of oxygen from the correct formula. Use oxidation numbers.
3. Calculate the percentage error.
4. List two sources of method error.

Questions* should I call it a mass or a balance.
Check the answers to #2 and 4
Should we take the masses of the things ^ before we heat them?