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Quantities in Chemical Reactions

This guide explores the principles of chemical reactions, emphasizing the law of definite proportions, which states that a specific compound always contains the same elements in fixed mass ratios. The text discusses how to calculate percent by mass using examples like water (H2O) and provides practice problems to reinforce understanding. By focusing on the conservation of mass, it also illustrates how to compare different compounds based on their mass proportions, essential for identifying chemical compounds.

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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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