Percentage composition is the percentage by mass of each element in a compound.
#"% by mass" = "mass of element"/"total mass of all elements" × 100%#
To find the % composition of #"CO"#, you calculate the % by mass of each element in a given mass of #"CO"#. It is simplest to use 1 mol of #"CO"#.
1 mol #"CO"# contains 1 mol of #"C"# and 1 mol of #"O"#.
28.01 g #"CO"# contain 12.01 g #"C"# and 16.00 g #"O"#.
#"% C by mass" = "mass of C"/"mass of CO" × 100 % = (12.01 color(red)(cancel(color(black)("g"))))/(28.01 color(red)(cancel(color(black)("g")))) × 100 % = 42.88 %#
#"% O by mass" = "mass of O"/"mass of CO" × 100 % = (16.00 color(red)(cancel(color(black)("g"))))/(28.01 color(red)(cancel(color(black)("g")))) × 100 % = 57.12 %#
