Percentage Composition

Percent Composition• Compounds are ALWAYS composed of elements in fixed

ratios. This is often referred to as the Law of Definite Proportions.

• This law states that elements are always present in the same proportions by mass in a compound. I.e. H2O is always 2H:1O any other proportions would not form water. H2O2 is hydrogen peroxide

• Since compounds always form in fixed proportions, we can calculate the Percent Composition of an element in a compound.

%Composition= x 100 = %

Mass of element Mass of compound

Example 1: A sample of a compound is analyzed and found to contain 0.90g of Ca and 1.60g of Cl. The sample has a mass of

2.50g. Find the % composition of the

compound.

Calculate the percentage composition of C3H7NO2

Step 1: Find the molar mass of C3H7NO2

M = 3(12.01) + 7(1.01) + (14.01) + 2(16.00) = 89.11 g/mol

Step 2: Find the % of each element by dividing the mass of each element by the total mass

%C = x 100 = 40.43 % %H = x 100 = 7.93%

%N = x 100 = 15.72%

%O = x 100 = 35.91 %

3(12.01) 89.11

7(1.01) 89.11

1(14.01) 89.11

2(16.00) 89.11

What is the percent composition of nitrogen in N2O?

Step 1: Find the molar mass of N2O

M = 2(14.01) + (16.00)

= 44.02 g/mol

Step 2: Find the % of each element by dividing the mass of each element by the total mass

%N = x 100 = 63.65%

%O = x 100 = 36.35 %

2(14.01) 44.02

1(16.00) 44.02

Calculate the mass of aluminium in 28 g of Aluminium Oxide

Step 1: Calculate the molar mass of aluminium oxide.

Al2O3

M = 2(26.98) + 3(16.00) = 101.96 g/mol

Step 2: Calculate the % mass of aluminium in aluminium oxide.

%Al = x 100 = 52.92%

2(26.98) 101.96

Step 3: Find the mass of aluminium in 28g of aluminium oxide.

mAl = x 28g = 15g 52.92 100

Alternate Solution = Al on top

Total on bottom

=mAl = 15 g

MAl

MTotal

mAl

mTotal

2(26.98) 101.96

mAl

28g