Transcript SCH3U 4.1:4.5 Law of Definite Proportions:Percent Compositi

Quantities in Chemical
Reactions
(4.1/4.5) Proportions in
Compounds and Percentage
Composition
Law of Definite Proportions
 a specific compound always contains
the same elements in definite
proportions by mass, regardless of how
it is synthesized
 compounds with the same mass
proportions must be the same
compound
 the proportions are found by calculating
the percent by mass.
Percent by Mass
(percentage composition)
 based on the law of conservation of
mass
MASS compd = sum of MASSES elements
 % by mass = MASSelement x 100%
MASScompd
Percent by Mass
 example: H2O
 made up of 2 atoms of hydrogen and 1
atom of oxygen
 to find percent by mass of each element:
H= (mass H / mass of water) x 100%
O= (mass O / mass of water) x 100%
Percent by Mass of H2O
Mass % of H =
=
mass of H (X2)
mass of H2O
1.01u X 2
X
1.01u X 2 + 16.00
=
2.02u X
18.02u
=
11.2%
Mass % of O = 100% - 11.2%
= 88.8%
100
X
100
100
Practice Problems
Q: A 78.0 g sample of an unknown
compound contains 12.4g of
hydrogen. What is the percent by
mass of hydrogen in the compound?
A: % Mass H = mass H x 100%
mass comp
= 12.4g x 100%
78.0g
= 15.9%
Practice Problems
Q: How many grams of oxygen can be
produced from the decomposition of
100.0 g of KClO3?
A: % mass O = mass O
x 100%
mass KClO3
= 3(16.00)u
x
100%
[39.10+35.45+3(16.00)]u
Practice Problems
Q: How many grams of oxygen can be
produced from the decomposition of
100.0 g of KClO3?
A (continued):
mass O
= %O x mass KClO3
= 0.3917 X 100.0g
= 39.17g
Practice Problems
Q: Two unknown compounds are tested.
Compound 1 contains 15.0g of
hydrogen and 120.0g oxygen.
Compound 2 contains 2.0g of
hydrogen and 32.0g oxygen. Are the
compounds the same?
HINT!! If % Masses are equal , then they are
the same
A: Compd 1%H =
=
%O =
=
[15.0 / (15.0+120.0)] x 100%
11.1%
[120.0 / (15.0+120.0)] x 100%
88.9%
Compd 2%H = [2.0 / (2.0+32.0)] x 100%
= 5.9%
%O = [32.0 / (2.0+32.0)] x 100%
= 94.1%
NOT THE SAME COMPOUNDS
Homework
Read pg. 160 – 162 & pg. 178 - 184
Finish “Percent Composition
Worksheet”
 pg. 184 “Section 4.5 Questions” #3 - 5