Percent Composition

Percent Composition: The percent in mass that a certain element in a compound contains.  In other words, the percentage of the molar mass that is taken up by a certain element.

          % Composition =    mass of element      *100
                                      mass of compound


What percentage of glucose (C6H12O6) is oxygen?

Molar mass of Glucose:
        6  C @ 12g/mol =     72g/mol
       12 H @   1g/mol =     12g/mol
        6  O @ 16g/mol =  + 96g/mol
                                        180g/mol

Calculate Percent Composition

  96g/mol    *100 = 53.3 %
180g/mol  

Now add the step.  How about if there is more then just one molecule?  Pull out you mole map because this is where you will need it!

What is the percent composition of 5.00g of Vitamin A (C20H30O)?

Step 1:  Molar mass:
      20  C  @  12g/mol = 240g/mol
      30  H  @    1g/mol =   30g/mol
        1  O  @  16g/mol = +16g/mol 
                                        286g/mol

Step 2:  Percentage of total molar mass
  240g/mol C     *100= 83.9%              30g/mol H     *100= 10.5%             16g/mol O     *100= 5.6%                  
286g/mol total                                    286g/mol total                                   286g/mol total

Step 3: Check your work
(always be sure to check that the percentages that you came up with add up to 100%)

  83.9%
  10.5%
 + 5.6%
  100.0%

Now one question is done.  Yes, I know it is a long process.  Going through all the steps is very helpful though and reminds you what is going on.  Do them even when it does start feeling tedious!


Now that you have seen some examples, try out this quiz!!!  Make sure you have some scrap paper and a periodic table at hand!

Percent Composition Quiz