2/3/2017
Percent Composition
can also calculate the percent
composition if you know only its
chemical formula
 The trick is to assume that you have
1 mole of the compound
 The formula is then used to calculate
the masses of each element
 We
Percent Composition II
Problem
 Propane
(C3H8), is one of the
lightest compounds obtained from
the fractional distillation of
petroleum
 Calculate the percent composition
of propane
Solution
Solution
 Known:
 Molar
mass of C = 12 g/mol
 Molar mass of H = 1 g/mol
 Molar mass C3H8 = 44 g/mol
 Unknown:
%
C?
 % H?
Solution
 Mass
of C in 1 mol C3H8 = 36 g
 Mass of H in 1 mol C3H8 = 8 g
 Molar mass C3H8 = 44 g/mol
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2/3/2017
Percent Composition
percent composition of a
compound is always the same,
regardless of the sample size!
 The
Percent Composition
 Propane
is 81.8% C and 18% H
 Therefore, in a 100g sample of
propane, there would be:
 (81.8%)(100g)
= 81.8g of C
 (18.0%)(100g) = 18g of H
Problem
Solution
A
compound is formed when
9.03 g Mg combines with
3.48 g N
 What is the percent composition of
this compound?
 How many grams of Mg and N
would be present in a 25 g sample of
this compound?
Problem
 72.2%
of 25 g =
 (0.722)(25 g) = 18.05 g Mg
 27.8%
of 25 g =
 (0.278)(25 g) = 6.95 g N
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