Sample Formal Lab Write-up
Your name
Title: Decomposition of potassium chlorate (KClO3)
Purpose: This lab will demonstrate the decomposition (breakdown) of KClO3 into KCl solid and
O2 gas when heated. The oxygen will be released into the atmosphere and the remaining KCl
can be measured to determine the change in mass. Using the Law of Conservation of Mass we
can determine the amount of oxygen released and calculate the percent of oxygen present in the
original sample.
Data:
Mass of empty dish
Mass of dish + KClO3
Mass of dish + KCl (after heating)
42.56 g
43.66 g
43.21
Calculations:
1. Mass of KClO3: 43.66 g -42.56 g = 1.10 g
2. Mass of KCl: 43.21 g -42.56 g = 0.69 g
3. Mass of O2: 1.10 g – 0.69 g = 0.41 g
4. Experimental % of oxygen using data:
g of oxygen
0.41
g of KClO3
1.10
X 100 =
37
%
5. Actual % of oxygen using mass numbers:
gfm of total oxygen
gfm of KClO3
48
X 100 =
39
%
122.6
6. Calculate your % error using calculations #4 and #5
(39% - 37%)/39% * 100 = 5.1% error
Analysis Questions:
1. What is the Law of Conservation of Mass? Conservation of mass means that matter
cannot be created or destroyed.
2. How do we use the Law to determine the amount of oxygen? Since the oxygen was part
of the original compound, it had mass. The change in mass had to come from the oxygen
being released. The difference in the masses of the sample before and after being heated
shows the amount of oxygen released.
3. What are possible sources of error in this lab? If the KClO3 was not heated enough, not all
of the oxygen would have been removed. If there were other compounds in the dish,
such as water, the loss of mass would have been greater than just oxygen.
4. Why does diatomic oxygen form when KClO3 is heated? Oxygen atoms are unstable
because they have unpaired electrons in their valence shells and will form diatomic
oxygen molecules which are more stable.
Conclusion: Decomposition is a type of chemical reaction in which a single reactant breaks
apart to form 2 or more new substances. When any chemical reaction occurs, the total mass of
the reactants must equal the total mass of the products. This is the Law of Conservation of
Mass. We can use this to calculate mass that is either lost or gained during a chemical reaction.
The oxygen is not destroyed, but rather is released as a gas to the air. Conservation of Mass
can also be demonstrated by using a balanced equation. In a balanced equation, the number of
atoms of an element has to be the same on both sides of the arrow. In this equation, for
example 2 KClO3  2 KCl + 3 O2 show 2 K, 2 Cl and 6 O on each side of the arrow. In this
experiment, our % oxygen was lower than the actual % oxygen in the formula. This means that
we probably did not heat the dish long enough for all of the KClO3 to fully decompose and that
there was some KClO3 left after we heated it. Next time we will makes sure to heat the sample
for the full amount of time at the hottest part of the flame.