How do you find percent composition from an empirical formula?

For each element, you divide the total mass of its atoms by the empirical formula mass and multiply by 100.

EXAMPLE

What is the of a compound with the empirical formula ##"CH"_2"O"##?

Solution

Step 1: Find the masses of the individual atoms.

##"C = 12.01 u"## ##"H = 1.008 u"## ##"O = 16.00 u"##

Step 2: Find the total mass of each atom in the formula.

##"1 C = 12.01 u"## ##"2 H = 2.016 u"## ##"1 O = 16.00 u"##

Step 3: Find the total mass of all the atoms in the formula.

##"CH"_2"O = (12.01 + 2.016 + 16.00) u = 30.03 u"##

Step 4: Find the mass percent of each atom.

##"mass %" = "mass of atom"/"total mass" × "100 %##

##"mass % C" = (12.01 color(red)(cancel(color(black)("u"))))/(30.03 color(red)(cancel(color(black)("u")))) × "100 %" = "40.00 %"##

##"mass % H" = (2.016 color(red)(cancel(color(black)("u"))))/(30.03 color(red)(cancel(color(black)("u")))) × "100 %" = "6.714 %"##

##"mass % O" = (16.00 color(red)(cancel(color(black)("u"))))/(30.03 color(red)(cancel(color(black)("u")))) × "100 %" = "53.29 %"##

Step 5: Check that your answers add up to 100 %.

##40.00 + 6.714 + 53.29 = 100.00##