Chapter 6, Section 2b
Percent Composition

You can calculate the mass percent of
elements in a given compound.
 The mass percent is the percent by mass of
a component of a mixture or of a given
element in a compound

Use the following formula:
Mass fraction mass of element present in 1 mol of compound
for a given =
compound
mass of 1 mol of compound
Percent Composition
If you are given the compounds name,
you will need to find the chemical
formula to calculate % comp
 In order to calculate % comp, you need
to know the molar mass of each element
in your compound and of your
compound
 The mass percentage of all the
elements in a compound must add to
100%.

Example

1.
Let’s calculate the mass percent of each
element in ethanol (C2H5OH).
Calculate the mass of each element.
1. Mass of C = 2 mol x 12.01 g/mol = 24.02 g
2. Mass of H = 6 mol x 1.01 g/mol = 6.06 g
3. Mass of O = 1 mol x 16.00 g/mol = 16.00 g
2.
Calculate the molar mass of ethanol
1. Mass of 1 mol C2H5OH = 24.02 g + 6.06 g +
16.00 g = 46.08 g = molar mass
Example Cont.
3.
Calculate the mass percent of each
element. Remember,
Mass fraction mass of element present in 1 mol of compound
for a given =
compound
mass of 1 mol of compound
1. Mass % C = 24.03 g C x 100 = 52.15%
46.08 g
2. Mass % H = 6.06 g H x 100 = 13.15%
46.08
3. Mass % O = 16.00 g x 100 = 34.72%
46.08
Example Cont
4.
Check your answer: 52.15 + 13.15 +
34.72 = 100.02%
Now You Try!
Calculate the percent composition of
sodium nitrate
1. Formula NaNO3
2. Calculate the mass of each element:

1. Na = 22.99 g, N = 14.01 g, O = 48.00 g
3.
Calculate the molar mass of sodium
nitrate:
1. Molar mass = 85.00 g
Now You Try Continue
3.
Calculate the mass percent of each
element:
1. Mass % Na = 27.05%
2. Mass % N = 16.48 %
3. Mass % O = 56.47 %
4.
Check your answer:
1. 27.05 + 16.48 + 56.47 = 100 %