42.88 % ##C## by mass and 57.12 % ##O## by mass
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 %##