Question

Question #b9857

Answer 1

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

Explanation:

EXAMPLE

What is the percent composition 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`