Ridgefield High School
PERCENT OXYGEN IN POTASSIUM CHLORATE
Background/Setup:
Chlorates
Chlorate, ClO31-, is a polyatomic ion. Polyatomic ions are groups of atoms that are covalently
bonded and have a charge. Ions that have a negative charge are called anions. Chlorate can form an
ionic bond with a positively charge ion (a cation) like a potassium ion, K 1+. The resulting ionic compound
is called potassium chlorate. Potassium chlorate, like other chlorates (sodium chlorate, lithium chlorate,
and others) can decompose upon heating. Chlorates decompose to produce the chloride compound of
the cation and oxygen gas. Potassium chlorate decomposes into potassium chloride and oxygen gas.
The word expression for this chemical reaction is:
Potassium chlorate yields potassium chloride and oxygen gas
The balanced equation for this chemical reaction is:
2 KClO3(s)  2 KCl(s) + 3 O2(g)
Note: (s) = solid,  means “yields”, and (g) = gas
Comparison of Potassium chlorate and Potassium chloride
Potassium chlorate is a white crystalline powder that has a melting point of 368.4 oC. Once
melted, it will begin to decompose at the slightly higher temperature of 400.oC. The resulting potassium
chloride that is produced upon decomposition is also a white crystalline powder but has a much higher
melting point of 776 oC. The other product, oxygen, is a gas at room temperature (and higher) and
dissipates into the air as the potassium chlorate decomposes and the potassium chloride is left behind.
Preparing a Crucible for Use
The decomposition of potassium chlorate requires the use of a crucible. A crucible is a ceramic
cup-shaped piece of lab equipment that has non-porous glazed surfaces inside and out except for the
bottom. The ceramic material is necessary because the decomposition takes place at a fairly high
temperature. The crucible (and its cover) must be clean and dry before adding the potassium chlorate
and heating. It is important to examine the crucible for cracks prior to use. A test for cracks can be made
by gently taping the crucible on the table and listening and feeling for vibrations. One must preheat the
crucible by heating gently at first then gradually increasing the heat for a period of 5 to 10 minutes before
being used. It is essential to preheat the crucible prior to use because during the cleaning phase water is
absorbed into the porous ceramic material through its bottom. The preheating phase is required to gently
drive out any water that was trapped within the walls of the crucible. If the water is not removed and the
crucible is quickly heated to a high temperature the water will vaporize and form a pressure within the
crucible walls that will crack the crucible as it expands rapidly. Even if the water does not crack the
crucible it will result in a lower mass after heating and produce poor results. So, it is essential to take time
at the beginning and properly prepare the crucible.
Crucibles must only be handled with crucible tongs, not beaker tongs. They may be heated on a
ring and ring stand and supported with a pipe-stem triangle. One must never place a hot crucible on a
balance. A hot crucible will not only damage the balance but will also produce convection currents above
it as the hot air rises and will cause fluctuations in the reading on the balance. The crucible will cool more
quickly if placed on a square of wire gauze. The wire gauze allows air to circulate under the crucible and
the metal wire absorbs and radiates the heat away from the hot crucible.
An Indirect Technique
In this experiment the mass and percentage of oxygen, a gas, must be determined. It would be
very difficult to determine the mass of a gas directly. An indirect technique would be much easier. If the
mass of the crucible and its contents are known before heating and then determined after heating, the
difference between them will equal the mass of the gas evolved during heating. Notice that if the mass of
the crucible is determined initially it can be used as tare and later deducted from all other calculations
without having to empty the contents out and determine their masses separately. Some of the mass of
materials is invariably lost in the transfer from container to container.
You are to experimentally determine the percent by weight of oxygen in a small sample of potassium
chlorate.