Mahopac High School
Chemistry
Determination of an Empirical Formula
and percent composition
The formula of a compound is the primary property of the compound that distinguishes it
from all other compounds. Each compound, regardless of the amount present, will
always have a set ratio, and percent composition, of the moles of each element present
within. These whole number ratios are used to determine the subscripts in the empirical
formula of the compound.
In this experiment you will show that magnesium and oxygen combine in a definite
whole number ratio by mass, and calculate the percent composition of oxygen in your
synthesis of the compound magnesium oxide.
Procedure : Record all observations as you perform the experiment
1) Cut a small piece of Mg ribbon.
2) If the surface of the Mg ribbon is not bright, use the edge of the scoopula or some
steel wool to scrape off the adhering film.
3) Mass the Mg sample to the nearest 0.01g.
4) Mass your crucible to the nearest 0.01g.
5) Wind the magnesium so it fits into the crucible, but not so it’s overlapping on
itself. Try winding it around and up a pencil.
6) Heat the crucible until the magnesium turns to a powder. CAUTION!!! DO
NOT stare directly at the bright light. It’s intensity is similar to a welders torch
and can damage your eyes after prolonged direct exposure.
7) Visually inspect the contents every so often to insure that all the Mg has reacted.
8) Allow the crucible to COOL.
9) Add enough distilled water to cover the product in the crucible.
10) GENTLY heat the crucible (AVOID BOILING OVER) to evaporate to dryness.
11) Repeat steps 8,9, and 10.
12) Calculate the mass the product in your crucible.
Calculations:
1)
2)
3)
4)
5)
6)
Calculate the mass of oxygen that reacted.
Calculate the percent oxygen in the product.
Calculate the % error of oxygen in your product.
Calculate the moles of Mg used in each trial.
Calculate the moles of oxygen reacted in each trial.
Find the ratio of moles of Mg to moles of O.
Conclusion questions:
1) Write the empirical formula of the oxide of magnesium based on your
calculations from each trial of your experiment.
2) If you had a large % error, what could account for this error?
3) Where did the oxygen come from that combined with the magnesium?
4) Could other gases have combined with the magnesium?