Percent composition of a compound refers to the mass percentages (also expressed as % w\w) of the constituent elements of the compound. It indicates the individual weights of the elements in any given weight of the compound.
For a pure compound, percent composition gives its elemental analysis; for an impure compound, percent composition gives the percent purity of the compound.
Self-Assessment Test
You may take this test before and/or after reading this text.
Calculate the percent by weight of ?
Calculation
If the individual weights of the constituent elements of a compound are known, then calculation of percent composition is straightforward.
Just divide the weight of each of the constituent elements by the weight of the compound and then multiply by 100:
`((weight of e\l\ement)/(weight of compound)) x 100 = % weight`
Example 1
A certain oxide of copper is found by experiment to contain 2.157 g of Cu and 0.543 g of O. Calculate its percent composition.
Step 1 To find the total weight of the compound, add the weights of the individual elements.
Step 2 In order to calculate the percent weights of the constituent elements, divide each weight of the elements by the weight of the compound and then multiply by 100.
`% Cu:`
`((2.157 g)/(2.700 g)) x 100 = 79.89%`
`% O:`
`((0.543 g)/(2.700 g)) x 100 = 20.11%`
Check that the total percentage is equal to 100.
If the weights are unknown, then percent composition can still be calculated from the known chemical formula of a compound.
It is made possible by considering the amounts of a compound and its components in terms of moles.
Example 2
Let's calculate the percent composition of ammonium sulfate, (NH4)2SO4:
Step 1 Determine the number of moles of the constituent elements contained in 1 mole of the compound.
1 mole of (NH4)2SO4 contains:
- 2 moles N
- 8 moles H
- 1 mole S
- 4 moles O
Step 2 Convert these number of moles to weight in grams by multiplying them by the molar weights of the elements.
`weight of N:`
`2 mo\l\es x (14.0067 g)/(mo\l\e) = 28.0134 g`
`weight of H:`
`8 mo\l\es x ( 1.0079 g)/(mo\l\e) = 8.0632 g`
`weight of S:`
`1 mo\l\e x (32.0666 g)/(mo\l\e) = 32.0666 g`
`weight of O:`
`4 mo\l\es x (15.9994 g)/(mo\l\e) = 63.9976 g`
Step 3 Add all the weights of the elements. This total is the weight of 1 mole of the compound. It is also known as the formula weight of the compound.
Step 4 Divide each weight of the elements by the formula weight of the compound.
`% N:`
`((28.0134 g)/(132.1408 g)) x 100 = 21.20%`
`% H:`
`((8.0632 g)/(132.1408 g)) x 100 = 6.10%`
`% S:`
`((32.0666 g)/(132.1408 g)) x 100 = 24.27%`
`% O:`
`((63.9976 g)/(132.1408 g)) x 100 = 48.43%`
Their total percentage must be equal to 100.
Uses
Percent composition is used to calculate the weight of a particular element in any given weight of a compound.
Example 3
Let's calculate the individual weights of the constituent elements present in 5.0 g of (NH4)2SO4.
Step 1 Find the percent composition of the compound.
As calculated in Example 2 , the percent composition of (NH4)2SO4 is:
Step 2 To determine the individual weights of the elements, multiply the mass percentage of each element by the given weight of the compound.
`weight of N:`
`5.0 g x 0.2120 = 1.1 g`
`weight of H:`
`5.0 g x 0.0610 = 0.3 g`
`weight of S:`
`5.0 g x 0.2427 = 1.2 g`
`weight of O:`
`5.0 g x 0.4843 = 2.4 g`
Step 3 Check that the total of the weights of the individual elements is equal to the given weight of the compound.
Percent composition is also used in the determination of the empirical formula of a compound. This is another topic that will be covered in a subsequent post.
