# Question #e2e8b

Jan 15, 2014

The percent composition is the percent breakdown of the relative element masses in the molar mass (gram formula mass) of a given molecular formula.

The percent composition of water can be found by first finding the molar mass of water.

Water has a ratio of two atoms of hydrogen for every one atom of oxygen ${H}_{2} O$

hydrogen has a mass of 1.01 amu
oxygen has a mass of 15.99 amu

H 2 x 1.01 = 2.02
O 1 x 15.99 = 15.99

2.02 + 15.99 = 18.01 g/mol

Hydrogen then has a percent composition determined by the division of 2.02 / 18.01 = 0.112 or 11.2%

Oxygen then has a percent composition determined by the division of 15.99 / 18.01 = .887 or 88.7%

Note that the percents only add up to 99.9%, due to the fact that we rounded off the decimal values.

To find the percent composition of a substance like aluminum hydroxide we first need to determine the formula for the molecule. In this case Aluminum is an ion of $A {l}^{+ 3}$ and the hydroxide is a polyatomic ion of $O {H}^{- 1}$. The formula is therefore $A l {\left(O H\right)}_{3}$.

aluminum has a mass of 27.9 amu
oxygen has a mass of 15.99 amu
hydrogen has a mass of 1.01 amu

Al 1 x 27.9 = 27.9
O 3 x 15.99 = 47.97 (distribute the subscript 3)
H 3 x 1.01 = 3.03 (distribute the subscript 3)

27.9 + 47.97 + 3.03 = 78.9 g/mol

Therefore, the percent composition breaks down as follows:

Al 27.9 / 78.9 = .354 or 35.4%
O 47.97 / 78.9 = .608 or 60.8%
H 3.03 / 78.9 = .038 or 3.8%

Here is a YouTube video for Percent Composition