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

% by mass = MASSelement x 100% MASScompd

MASS compd = sum of MASSES elements

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 H2OMass % of H = mass of H (X2) X 100

mass of H2O

= 1.01u X 2 X 100 1.01u X 2 + 16.00

= 2.02u X 10018.02u

= 11.2%

Mass % of O = 100% - 11.2%= 88.8%

Practice ProblemsQ: 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 ProblemsQ: 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

= 39.17%

Practice ProblemsQ: 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 ProblemsQ: 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

HomeworkRead pg. 160 – 162 & pg. 178 - 184 Finish “Percent Composition Worksheet” pg. 184 “Section 4.5 Questions” #3 - 5