Percent Composition
(from formula)
Percent composition refers to what percent of the total mass of a
compound comes from a specific element.
Going from a molecular formula to a percent (or %) composition
is easy and is what I will work on here. Going in the opposite
direction, that is from a % composition to a molecular formula can
be much more difficult, and is the subject of a different series of
tutorials.
Percent Composition
(from formula)
Just like any % calculation it is going to look like this
Part/total x 100%
In this case the ‘part’ is the mass of any one element in a compound
and the ‘total’ is the total mass of the compound.
Putting this together you have:
% composition of an element =
total mass of one element in compound
× 100%
total mass of compound
Percent Composition
(from formula)
Let’s start with the compound Ca(OH)2.
What is the % composition of Ca, O, and H in this compound?
Well, from the formula you know that 1 mole of this compound
contains 1 mole of Ca, 2 moles of O and 2 mole of H. Thus one
mole of this compound would contain
1 mole x 40.08 g or 40.08 g of Ca
2 moles x 16.00 or 32.00 g of O
2 moles x 1.008 or 1.016 g of H
And your molecular mass, is simple the total of these masses or
1(40.08) + 2(16.00) + 2(1.008) = 74.096 g
Percent Composition
(from formula)
% composition of Ca is then:
(40.08g/74.096g) x 100% = 54.09%
% composition of O is:
(32.00g/74.096g) x 100% = 43.19%
And % composition of H is:
(2.016g/74.096g) x 100% = 2.72%
The sum of the %’s should = 100 so this is a quick check
54.09 + 43.19 + 2.72 = 100.00% so you are good!
Percent Composition
(from formula)
Let’s try another example. What is the % composition of all of
the elements in the compounds NaHCO3?
Percent Composition
(from formula)
NaHCO3
Mass of Na = 1 x 22.99g = 22.99g
Mass of H = 1 x 1.008g = 1.008g
Mass of C = 1 x 12.01g = 12.01g
Mass of O = 3 x 16.00g = 48.00 g
TOTAL
= 84.008g
% comp of Na = 22.99g/84.008g x 100% = 27.37%
% comp of H = 1.008g/84.008g x 100% = 1.20%
% comp of C = 12.01g/84.008g x 100% = 14.30%
% comp of O = 48.00g/84.008g x 100% = 57.14%
(Check -sum of %s is 100.01, close enough!)
Percent Composition
(from formula)
Finally try one more example, what is the % composition of all
the elements in Iron(III) nitrite?
Percent Composition
(from formula)
The correct formula here is
Fe(NO2)3
Percent Composition
(from formula)
Fe(NO2)3
Mass of Fe = 1 x 55.85g = 55.85g
Mass of N = 3 x 14.01g = 42.03g
Mass of O = 6 x 16.00g = 96.00g
TOTAL
= 193.88g
% comp of Fe = 55.85g/193.88g x 100% = 28.81%
% comp of N = 42.03g/193.88g x 100% = 21.68%
% comp of O = 96.00g/193.88g x 100% = 49.52%
(Check - sum of %s is 100.01, close enough!)