Percentage Composition
% Composition is the % by mass of each element in a
compound.
Ex: Find the percentage composition of sodium in sodium
chloride:
Na+
Cl - :
formula is NaCl
1 mol Na x 22.99 g Na = 22.99 g Na
1 mol Na
1 mol Cl x 35.45 g Cl = 35.45 g Cl
1 mol Cl
molar mass NaCl = 58.44 g/mol
22.99 g Na
x 100 = 39.34 % Na
58.44 g NaCl
Find the percentage composition of sodium nitrate.
Na+
NO3 - : formula is
NaNO3
Don’t forget to
multiply mass
by #of Moles
1 mol Na x 22.99 g Na = 22.99 g Na
1 mol Na
1 mol N x 14.01 g N = 14.01 g N
1 mol N
3 mol O x 16.00 g O = 48.00 g O
1 mol O
molar mass NaNO3 = 85.00 g/mol
22.99 g Na
x 100 = 27.05 % Na
85.00 g NaNO3
14.01 g N
x 100 = 16.48 % N
85.00 g NaNO3
48.00 g O
x 100 = 56.47% O
85.00 g NaNO3
Once you find molar
mass, you are just
dividing the part
(element) by the whole
(compound) to get the
percent composition
Why mass instead of moles? Isn’t 2/3 of the water molecule
hydrogen?
 Moles indicate the amounts of each atom needed to
make the molecule stable from an electron
standpoint.

2.02 g H
x 100 =
18.02 g H2O
11.21 % H