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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

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quantities in chemical reactions

Quantities in Chemical Reactions

(4.1/4.5) Proportions in Compounds and Percentage Composition

law of definite proportions
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
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
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 h 2 o
Percent by Mass of H2O

Mass % 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 100

18.02u

= 11.2%

Mass % of O = 100% - 11.2%

= 88.8%

practice problems
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 problems1
Practice Problems

Q: How many grams of oxygen can be produced from the decomposition of 100.0 g of KClO3?

A: % mass O = mass Ox 100%

mass KClO3

= 3(16.00)ux 100%

[39.10+35.45+3(16.00)]u

= 39.17%

practice problems2
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 problems3
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

slide10
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
Homework
  • Read pg. 160 – 162 & pg. 178 - 184
  • Finish “Percent Composition Worksheet”
  • pg. 184 “Section 4.5 Questions” #3 - 5