Percent Composition

Percent composition is used to discover what percentage of the compound a certain element makes up, and it is actually quite easy to find. You just need to follow easy steps:

1. calculate the molar mass of the compound
2. find the molar mass of each element. Then divide the compound's molar mass by the element's molar mass
3. multiply by 100 to get the percentage (round to the nearest tenth!)

                               mass of element    
                             mass of compound    x 100  = % composition

Ex. What's the percent comp. of the compound Na₂O?

Molar mass of Na -> 23.0 g x 2 atoms = 46 g
Molar mass of O -> 16 g
Molar mass of Na₂O -> 46 g + 16g = 62 g

Next step is to divide each element mass by the mass of the compound:

46 g
62 g  = 0.741                        0.741 x 100 = 74.1 %

The percentage of Na 74.1%

16 g
62 g  = 0.258                        0.258 x 100 = 25.8 %

The percentage of O is 25.8 %

74.1% + 25.8% = 99.9%

**When you add the final percentages together, they should equal ~100 %. That's how you know your answers right!**