Question #b9857

1 Answer
Jan 16, 2014

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"CH2O?

Solution

Step 1: Find the masses of the individual atoms.

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

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

"1 C = 12.01 u"1 C = 12.01 u
"2 H = 2.016 u"2 H = 2.016 u
"1 O = 16.00 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"CH2O = (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 %=mass of atomtotal 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