Percentage Composition

Percentage Composition

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  • Finding Percent Composition is very simple.
  • You first begin by finding all of the molar masses of each element in a compound. 
  • The compound we will use in this example is CO2. C (carbon) has a molar mass of 12.01. O (oxygen) has a molar mass of 16.0. Because there are 2 Oxygen atoms per Carbon atom, the molar mass of Oxygen in this compound is 32.
  • We then add the masses together to find the total mass of the molecule. 12.01 of carbon and 32.0 of oxygen equals 44.01 moles.
  • There is 72.71% Oxygen in the compound CO2.
  • Lets start with oxygen. There is 32 moles of Oxygen in the 44.01 moles of CO2. 32 divided by 44.01 is .7271, then you multiply by 100 to get 72.71%
  • The next step is to find out how much of each element is in the molecule. 
  • To find the percentage of carbon in CO2, you follow the same steps as you did with oxygen
  • 12.01 moles of carbon divided by 44.01 moles of CO2 is equal to .2728, which is then multiplied by 100 to get 27.28% of carbon in the compound CO2.
  • A simple tip to check if you got the right answer is to see if the percentages you got add up to 100%.
  • 72.71% of carbon plus 27.28% of oxygen equals 99.99%, which is really close to 100%, so we got the calculations correct!
  • So remember... (number of atoms of element)(molar/atomic mass) divided by (total molar mass of compound) gives you the percent composition!
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