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Quantities in Chemical

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(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

MASS compd x 100%

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) 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

Read pg. 160

– 162 & pg. 178 - 184

Finish

“Percent Composition

Worksheet

”

pg. 184

“Section 4.5 Questions” #3 - 5