Percent composition

Percent composition is a measure of the mass of each element is present in a compound. The percentages of the elements in a compounds always add up to approximately or exactly 100%. (You can use this tip to check your answers.)

How do you calculate the percent composition of an element in a compound?
Step 1: Find out the total molar mass of the compound
Step 2: Find out the molar mass of each element in the formula
Step 3: Percent Composition = mass of element/ mass of compound x 100%
Step 3: Check if the percentages add up to approximately 100%. If not, the answer is incorrect.


Let's do an example by following these steps.
eg. What is the percent composition of each element in Carbon Dioxide?

Carbon Dioxide = CO2  (MM is 44)C's molar mass = 12
O's molar mass = 16 x 2 = 32 (multiplied by 2 because 2 O's present)

C's percent com. = 12/44 x 100% = 27%
O's percent com = 32/44 x 100% = 73%

Check: 27%+ 73% = 100% <----- When they add up to 100%, it's correct.

Watch this online presentation and try the problems to see if you understand!
http://www.wisc-online.com/objects/ViewObject.aspx?ID=GCH7104


- Catherine