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Do Now

Percent Composition I
What volume do these gases
occupy at STP?
3.20 x 10-3 mol CO2
 3.70 mol N2

Percent Composition
Solution
way to express the relative
amounts of elements in a
compound is:
 One
=
7.17 x 10-2 L CO2
percent composition (by mass) of
each element in the compound
A
 This
=
82.9 L N2
is how the composition of
most medications are expressed
Percent Composition
need to compute a percent
composition for each element
in a compound
 You
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Percent Composition
example, the percent composition
of K2CrO4 is:
 For
 K:
40.3%
 Cr:
26.8%
32.9%
 O:
 ------------- 100%
 These
percents must total 100%
Problem
Percent Composition
percent composition is the mass
or volume of the part divided by the
mass or volume of the whole,
multiplied by 100
 The
 When
a 13.60 g sample of a
compound containing only
Mg and O is decomposed,
5.40 g of O is obtained
 What is the percent composition
of each element in this
compound?
Solution
Solution
 Known:
 Mass
compound = 13.60 g
 Mass O = 5.40 g
 Mass Mg = Mass compound – Mass O
= 13.60 g – 5.40 g = 8.20 g Mg
 Unknown:
%
Mg?
 % O?
 Strategy:
 Use
the formula to compute the
% comp for each of the 2 elements
in this compound
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Solution
Solution
 Check
your answer!
 The percentages should add up
to 100%
 60.3% + 39.7% = 100%
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