Unit 5.3 Percent Composition
Is there anything healthy in this jar? Packaged
foods that you eat typically have nutritional
information provided on the label. The label on a jar
of peanut butter (shown above) reveals that one
serving size is considered to be 32 g. The label also
gives the masses of various types of compounds that
are present in each serving. One serving contains 7
g of protein, 15 g of fat, and 3 g of sugar. By
calculating the fraction of protein, fat, or sugar in
one serving of size of peanut butter and converting
to percent values, we can determine the composition
of the peanut butter on a percent by mass basis.
Percent Composition
Chemists often need to know what elements are present in a compound and in what percentage.
The percent composition is the percent by mass of each element in a compound. It is calculated in a
similar way that we just indicated for the peanut butter.
% by mass =
mass of element
(100)
( mass
)
of compound
Percent Composition from Mass Data
The sample problem below shows the calculation of the percent composition of a compound based on mass
data.
Sample Problem One: Percent Composition from Mass
A certain newly synthesized compound is known to contain the elements zinc and oxygen. When a 20.00 g
sample is decomposed, 16.07 g of zinc remains. Determine the percent composition of the compound.
Step 1: List the known quantities and plan the problem.


mass of compound = 20.00 g
mass of Zn = 16.07 g
Subtract to find the mass of oxygen in the
compound. Divide each element’s mass by the
mass of the compound to find the percent by
mass.
Unknown


percent Zn = ? %
percent O = ? %
Step 2: Calculate.
Mass of oxygen = 20.00 g sample – 16.07 g Zn = 3.93 g O
% Zn = (16.07 g Zn / 20.00 g sample) (100) = 80.35 % Zn
% O = (3.93 g O / 20.00 g sample) (100)
= 19.65 % O
Step 3: Think about your result.
The calculations make sense because the sum of the two percentages adds up to 100%, that all of the
mass is accounted for in the percentages.
80.35 % Zn + 19.65 % O = 100 % Total
By mass, the compound is mostly zinc.
Percent Composition from a Chemical Formula
The percent composition of a compound can also be determined from the formula of the compound. The
subscripts in the formula are first used to calculate the mass of each element, and the total molar mass
in one mole of the compound. The mass supplied by an element is divided by the molar mass of the
compound and multiplied by 100%. The % composition is determined for each element.
% by molar mass =
(
mass of element in 1 mol
Molar mass of compound
)(100)
The percent composition of a given compound is always the same as long as the compound is pure.
Sample Problem Two: Percent Composition from Chemical Formula
Dichlorineheptoxide (Cl2O7) is a highly reactive compound used in some organic synthesis
reactions. Calculate the percent composition of dichlorineheptoxide.
Step 1: List the known quantities and plan the problem.



mass of Cl in 1 mol Cl2O7 = (2) (35.45 g Cl) = 70.90 g Cl
mass of O in 1 mol Cl2O7 = (7) (16.00 g O) = 112.00 g O
molar mass of Cl2O7 = 70.90 g + 112.00 g = 182.90 g/mol
Calculate the percent by mass of each element by dividing the
mass of that element in 1 mole of the compound by the molar mass
of the compound and multiplying by 100%.
Unknown


percent Cl = ? %
percent O = ? %
Step 2: Calculate.
% Cl = (70.90 g Cl / 182.90 g sample) (100) = 38.76 % Cl
% O = (112.00 g O / 182.90 g sample) (100) = 61.24 % O
Step 3: Think about your result.
Remember, if you have accounted for all of the atoms in the compound, the percentages add up to 100%.
38.76 % Cl + 61.24 % O = 100 % Total
Percent composition can also be used to determine the mass of a certain element that is contained in any
mass of a compound. In the previous sample problem, it was found that the percent composition of
dichlorineheptoxide is 38.76% Cl and 61.24% O.
Suppose that you needed to know the masses of chlorine and oxygen present in a 12.50 g sample of
dichlorineheptoxide. You can set up a conversion factor based on the percent by mass of each element.
Use a fraction representing the number of grams of the element in a 100 g sample as the conversion
fraction.
(12.50 g Cl2O7) (38.76 g Cl / 100 g sample) = 4.845 g Cl
(12.50 g Cl2O7) (61.24 g O / 100 g sample) = 7.655 g O
The sum of the two masses is 12.50 g, the mass of the sample size.
Summary


% composition of element can be accomplished based on mass.
% composition of element can be accomplished based on molar mass..
Practice
Use the link below to review material and do calculations. Read both parts of the lesson and do as many
calculations as you have time for.
http://www.chemteam.info/Mole/Percent-Composition-Part1.html
Review
1.
What is the formula for calculating percent composition by mass?
2.
What information do you need to calculate percent composition by mass?
3.
What is the formula for calculating percent composition by molar mass?
4.
What do subscripts in a chemical formula tell you?
5.
What must all of the percentages calculated for a compound add up to?
Answers
1.
% composition by mass = (mass of element / mass of compound)(100)
2.
Mass of compound, and mass for elements.
3.
% composition by malar mass = (mass of element in 1 mol/molar mass of compound)(100)
4.
Subscripts in a formula tell the number of that type of atom in the molecule.
5.
If claculations are done right, the percentages for all elements in a compound must add up to
100% (or close to it if any number rounding has been done).