Quantities in Chemical
Reactions
(4.1/4.5) Proportions in
Compounds and Percentage
Composition

 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 .

 based on the law of conservation of mass
MASS compd
= sum of MASSES elements
 % by mass = MASS element x 100%
MASS compd

 example: H
2
O
 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%

2
Mass % of H = mass of H (X2) X 100 mass of H
2
O
= 1.01u X 2 X 100
1.01u X 2 + 16.00
= 2.02u
X 100
18.02u
= 11.2%
Mass % of O = 100% - 11.2%
= 88.8%

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 KClO
3
?
A: % mass O = mass O x 100% mass KClO
3
= 3(16.00)u x
100%
[39.10+35.45+3(16.00)]u
= 39.17%

Practice Problems
Q: How many grams of oxygen can be produced from the decomposition of
100.0 g of KClO
3
?
A (continued): mass O = %O x mass KClO
3
= 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 = [15.0 / (15.0+120.0)] x 100%
= 11.1%
%O = [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