# 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

.

### (percentage composition)

 based on the law of conservation of mass

MASS compd

= sum of MASSES elements

% by mass = MASS element

MASS compd x 100%

### Percent by Mass

 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

### O

Mass % of H = mass of H (X2) mass of H

2

O

X 100

= 1.01u X 2

1.01u X 2 + 16.00

X 100

= 2.02u

18.02u

X 100

= 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

– 162 & pg. 178 - 184

Finish

“Percent Composition

Worksheet

 pg. 184

“Section 4.5 Questions” #3 - 5