Download Percent Composition

% 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
A Substance is Produced & the Formula
of the Compound is Unknown…
• The composition of the compound based
on mass is determined by experiment
1)
• We call this…
percent composition by mass of a
substance
•
The mass amounts are converted
into percentages to give us the
percent composition
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
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
in the compound
= mass given of the element
mass given of the compound
• For example we can react Sodium and
Chlorine gas to form Sodium Chloride
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%
8.78g
% of Cl- = 5.33g x 100%
8.78g
% Na+ = 39.3 %
% of Cl- = 60.7%
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.
Check answer on pg 178 of
your text
(Nelson 11)
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
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)
Let’s determine the percentage composition
by mass of Li2SO4
GIVEN: mLi = 6.95 u x 2 = 13.90 u
mS = 32.01 u x 1 = 32.01 u
mO = 16.00 u x 4 = 64.00 u
mLi2SO4 = 109.91 u
% Li = 13.90 u x 100%
109.91 u
% Li = 12.65 %
% S = 32.01 u x 100%
109.91 u
% S = 29.12 %
% of O = 64.00 u x 100% = 58.23 %
109.91 u
Have we done our math
correctly???
% Li = 12.65 %
% S = 29.12 %
% O = 58.23 %
=
Have we done our math
correctly???
% Li = 12.65 %
% S = 29.12 %
% O = 58.23 %
=
Yes, we did it!!!
Empiracle Formula
• http://www.khanacademy.org/science/che
mistry/chemical-reactionsstoichiometry/v/another-masscomposition-problem