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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. 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

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

  2. 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.

  3. 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

  4. 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%

  5. 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%

  6. 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%

  7. 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%

  8. 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

  9. 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

  10. 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

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

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