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# 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##