Science project

Determining the Empirical Formula of Potassium Chlorate through Thermal Decomposition

Research Question:

  • What is thermal decomposition?
  • What happens when KClO3 thermally decomposes?
  • How can you calculate the formula for a reaction based upon the weights of the reactants?

Potassium chlorate (KClO3) is a powerful oxidizer. Oxygen is released when this chemical is thermally decomposed. In this experiment, students will learn how to calculate the formula for the thermal decomposition of KClO3 by comparing the weights of a sample of potassium chlorate before and after it thermally decomposes.


  • Potassium chlorate (KClO3)
  • Ceramic crucible
  • Ring stand and Bunsen burner
  • Good laboratory scale

Experimental Procedure:

  1. Preheat the crucible for 5 minutes to remove any moisture.
  2. Weigh the crucible.
  3. Weight out 2.5 grams of KClO3 and place in the crucible.
  4. Weight the crucible with the KClO3 in it.
  5. Place the crucible and KClO3 under a moderate flame for 20 minutes. Take notes periodically as to the state of the KClO3. Once it stops bubbling, turn off the flame. At this point the KClO3 has thermally decomposed to potassium chloride
  6. Allow the crucible to cool for five minutes.
  7. Weigh the crucible and potassium chloride (KCl).
  8. Repeat steps 1-7 two more times for a total of three trials. Create a table that shows the weight of (A) the empty dish, (B) the empty dish and the KClO3 and (C) the empty dish and KCl. Calculate the average of these for all three trials so that you have the average weights for (A), (B), and (C). Include in your table the weight of the initial KClO3 and weight of the KCl. Calculate the weight of the missing oxygen. Calculate the percentages of oxygen and KCl. Using these values, calculate the molar ratio of KClO3 to KCl to O2. Using the molar values, calculate the balanced chemical equation for the reaction.

Terms/Concepts: oxidizer, thermal decomposition, stoichiometry, moles

References: • Vonderbrink, S.A. Laboratory Experiments for AP Chemistry. 2nd Edition. (Batavia, IL: Flinn Scientific, Inc. 2006)

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