In order to calculate the percent composition of a compound, you need to determine the weights of its constituent elements. For details and examples, see my previous post Percent Composition.
After determining the weight percentages of a compound's constituent elements, you can now calculate the individual weights of the elements contained in any given weight of that compound.
Percent Composition Calculator does just that. It provides you a quick check on your calculations whether you're solving or composing percent composition problems.
Just give the correct formula, weight and percent purity (if any) of the compound:
Formula. For the most commonly encountered binary inorganic compounds, a message is provided below the formula entry line if you enter their incorrect formula.
Weight. You can use any unit for the weight, meaning if your unit of weight is in grams then the computed weights of the compound's components will also be in grams.
% Purity. It is optional. If you don't enter a value, the application assumes that the compound is pure.
You need to round off the results to the required number of significant digits.
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.
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.
