1. % Percent Composition
There are two situations where we calculate percent composition:
1) When a substance is produced and we do not know the formula of a compound
2) When we already know the formula of a compound

2. A Substance is Produced & the Formula of the Compound is Unknown…
The composition of the compound based on mass is determined by experiment
We call this…
percent composition by mass of a substance
The mass amounts are converted into percentages to give us the percent composition 1)

3. Examples
If you bought some x-mas jelly beans that came in green and red, you could mass out each colour and find the % composition by mass….
ex. 20g of red jelly beans
30g of green jelly beans
50g = TOTAL mass of all the jelly beans
Therefore, you would have…
20g/50g x 100% = 40% by mass of red jelly beans
and
30g/50g x 100% = 60% by mass of green jelly beans

4. Chemical Reactions & Percent Composition
In chemistry we can mass the reactants and products in a synthesis reaction and use those values to determine the percentage of each element in a compound assuming the reaction involves reactants that are pure elements.
Mass of element = mass given of the element
in the compound mass given of the compound
For example we can react Sodium and Chlorine gas to form Sodium Chloride

5. Sample Problem using NaCl
Reactive sodium metal, Na(s) is reacted with the poisonous green chlorine gas, Cl2(g)
2 Na(s) + Cl2(g)  2 NaCl(s)
If 3.45 g of Na reacted with 5.33 g of Cl2 gas to give
8.78 g of NaCl, what is the percentage composition by mass of sodium chloride…
GIVEN: mNa+ = 3.45g mCl- = 5.33g mNaCl = 8.78g
% Na+ = 3.45g x 100% % of Cl- = 5.33g x 100%
8.78g g
% Na+ = % % of Cl- = 60.7%

6. We know the answers are correct because the percentages add up to 100% (39.3% + 60.7%)
Your Turn….
Determine the percent composition of water by mass if 2.5 g of hydrogen completed reacted with oxygen to form 22.5 g of water.
NOTE: To determine the mass of oxygen gas that was used in the reaction subtract the mass of hydrogen from the mass of the water.

7. Check answer on pg 178 of your text
(Nelson 11)

8. When the Formula of a Compound is Known
When we already know the formula of a compound then we can use percent composition based on the formula and compare it to values determined experimentally (by reacting the sample) to see how pure the sample was that was used in the reaction.
We can also use the known formula to compare percent composition values to see if the experiment was done with accuracy
Percent composition is used in fertilizers. If the percent composition of nitrogen is determined, then the amount that should be applied to the lawn can be calculated

9. Calculating Percent Composition Using the Chemical Formula
If you are given a name of a compound, then you will need to determine the correct chemical formula
potassium nitrate  KNO3
these questions will not give you values for mass, instead we use the atomic mass unit or molar mass for each element (provided by the periodic table)

10. Let’s determine the percentage composition by mass of Li2SO4
GIVEN: mLi = u x 2 = u
mS = u x 1 = u
mO = u x 4 = u
mLi2SO4 = u
% Li = u x 100% % S = u x 100%
u u
% Li = % % S = %
% of O = u x 100% = % u

11. Have we done our math correctly???
% Li = %
% S = %
% O = % =

12. Have we done our math correctly???
% Li = %
% S = %
% O = % =
Yes, we did it!!! %

13. Empiracle Formula