Percent Composition Lab
Introduction: Percent is determined by dividing the part by the whole and multiplying by one hundred. The theoretical
percentage of an element in a compound is determined by dividing the mass of the element in the compound by the total
mass of the compound. For example, to determine the percent of oxygen in sodium carbonate, Na2CO3, first determine the
molar mass of the compound
2Na + lC + 3O = 2(23.00 g/mol) + l(12.01 g/mol) + 3(l6.00 g/mol) = 106.01 g/mol
Then divide the mass of oxygen in the formula, 48.00 g/mol, by the molar mass, l06.01 g/mol.
(48.00/106.01) x 100 = 45.42% O
In this experiment you will determine experimentally the percent of oxygen in the compound potassium chlorate. When
heated potassium chlorate will decompose into potassium chloride and oxygen according to the following equation:
KClO3(s) → KCl(s) + O2(g)
Finding the mass of potassium chlorate present before the reaction and the mass of potassium chloride after the reaction
the mass of the oxygen released can be calculated.
To determine the accuracy of your result you compare this experimental value to the theoretical percent of oxygen in
potassium chlorate.
Safety: Avoid contact with potassium chlorate. Wash hands after lab.
PreLab Questions:
______________1. Theoretically calculate the percent of oxygen in potassium chlorate. This is known as the theoretical
yield. Show work.
______________2. Find the empirical formula for a compound that contains 32.8% chromium and 67.2%
chlorine. Show work.
Procedures:
1. Mass a clean dry crucible and cover on the balance. Record this mass on the data table. Leave the crucible on the
balance and add between 2 to 2 ½ grams of potassium chlorate to the crucible. Record the mass of the crucible
and potassium chlorate on the data table.
2. Set up a ring stand with an iron ring and a clay triangle to support the crucible. Place the crucible in the clay
triangle, and heat with the Bunsen burner. To control the heat, hold the burner in your hand. Heat slowly at first
and gradually increase the intensity of the heat. Finally heat continuously with the hottest part of the flame for at
least 5 minutes. Next to your name write what you like most about school.
3. Allow the crucible to cool for approximately 10 minutes. When cool enough to pick up, mass the crucible and its
contents. Rinse the crucible with tap water and dry it.
4.
Data:
1. Mass of empty crucible and cover
2. Mass of crucible and cover with potassium chlorate
3. Mass of potassium chlorate (#2-#1)
4. Mass of crucible and cover and product after heating
5. Mass of oxygen (#2-#4)
Name:
