Percent Composition

• The percentage of mass of an element in a compound always the same.
• To find the percent by mass, determine the mass of each element present in one mole.

Examples:

1.Determine the percent by mass of  Carbon and oxygen in Carbon dioxide?

C= 12.01

O= 16.0

Find how many grams there are in each element in 1 mole of CO2.

(1 moles of Carbon and 2 moles of oxygen)

C= 12.01 x1= 12.01

O= 16.00 x2= 32.0

Find the total mass of 1 mole:

The total mass of CO2 is.

12.01+32.00=44.01g

Find the mass percentage :

Mass percentage of C= (12.01 g/ 44.01g)x100= 27.29%

Mass Percentage of O= (16.00 g/44.01 g)x100=72.71%

2. Determine the percentage by mass of the element Carbon in Carbon Dioxide (CO2).

Find the total mass of CO2.

C= 12.01 x1= 12.01

O2 = 16.00 x2= 32.0

12.01+32.00= 44.01

Find the mass of C.

C=12.01 

Find the percentage by mass of C. 

Mass of C=                12.01g

Total mass of CO2=   44.01g

12.01g/44.01g = 0.27 

0.27 x 100= 27%