SCH 3U
Moles
Name: _________________________
Percentage Composition
Percentage composition is the percent, by mass, of each element in a compound. This value gives the fraction of each
element in a compound and can be calculated 2 ways.
1. Using Experimental Data:
With this method, the total mass of a sample of a compound must be known as well as the mass of each
element. To calculate the percent composition of an element, use the formula below.
Example:
A 10.5 g sample of glucose was combusted to yield 4.20 grams of carbon, 5.60 grams oxygen and 0.70 grams of
hydrogen. Calculate the percent composition for each element in the glucose sample.
Solution:
2. Using the Chemical Formula of a Compound:
With this method the molar mass of the compound must be calculated and the total mass of each element (# of
atoms on an element multiplied by the atomic mass of the element). To calculate the percent composition of an
element, use the formula below.
Example:
Determine the percent composition of each element in phosphoric acid (H3PO4).
Solution:
MM of H3PO4= 3 H + P + 4 O
MM of H3PO4= 3(1.01 g/mol) + 30.97 g/mol + 4(16.00 g/mol)
MM of H3PO4=98.00 g/mol
SCH 3U
Moles
Exercise:
Name: _________________________
Complete the following on a separate piece of paper and express the answer to the correct number of significant digits.
1. When 2.00 g of lead is heated with sulfur, 2.30 g of lead (II) sulfide is formed. Calculate the percent composition
by mass of each element in lead (II) sulfide.
[87.0 % Pb, 13.0 % S]
2. Mercury (II) oxide decomposes to its elements when heated. Use the data below to calculate the percent
composition of each element.
[92.30 % Hg, 6.70 % O]
mass of test tube
mass of test tube and mercury (II) oxide
mass of test tube and mercury
12.25 g
13.68 g
13.57 g
3. Iron and sulfur react to form iron (II) sulfide. Use the data below to calculate the percent composition of each
element.
[63.6% Fe, 36.4% S]
mass of crucible
mass of crucible and iron
mass of crucible and iron (II) sulfide
30.6 g
41.8 g
48.2 g
4. A compound contains 30.1 % oxygen by mass. What mass of oxygen is there in 5.0 g of the compound? [1.51 g]
5. Epsom salt is the hydrated form of magnesium sulfate and can be dehydrated by heating. Use the following data
to calculate the percent of water in Epsom salt.
[51.1 %]
mass of crucible
mass of crucible and Epsom salts
mass of crucible and dry magnesium sulfate
40.88 g
65.52 g
52.92 g
6. Calculate the percent composition by mass of all elements in each compound
a) H2O2
[5.91% H, 94.09 % O]
f) HNO3
b) CO
[42.89% C, 57.11 % O]
g) Pb(NO3)2
c) Na2SO4
h) SiO2
d) H2SeO4
i) C6H12O6
e) H2CO3
j) SnO2
7. An unknown compound has a molar mass of 18.02 g/mol. It is known to contain 88.9% oxygen and 11.1 %
hydrogen. What is the compound?
[H2O]
8. An unknown compound contains 7.17 % oxygen and 92.83% Pb. If the molar mass of the compound is 223.2
g/mol, what is the formula and name of the compound?
[PbO, lead (II) oxide]
9. An unknown compound contains 69.55 % oxygen and 30.45 % nitrogen. If the molar mass of the compound is
92.02 g/mol, what is the formula and name of the compound?
[N2O4]