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Unit 2: Basic Chemistry Skills Lesson 4: Empirical and Molecular Formulas

Unit 2: Basic Chemistry Skills Lesson 4: Empirical and Molecular Formulas. Do Now: Determine the percent oxygen in water, H 2 O. Currently Playing: 3 Doors Down - Kryptonite Book Section: 3.5.

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Unit 2: Basic Chemistry Skills Lesson 4: Empirical and Molecular Formulas

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  1. Unit 2: Basic Chemistry SkillsLesson 4: Empirical and Molecular Formulas Do Now: Determine the percent oxygen in water, H2O. Currently Playing: 3 Doors Down - Kryptonite Book Section: 3.5

  2. One can calculate the empirical formula of a compound from either mass data or percent composition. • An empirical formula is the lowest whole-number ratio of atoms in a compound. • Ex: What is the empirical formula of butane, C4H10? • C2H5 Calculating Empirical Formulas

  3. The compound para-aminobenzoic acid (you may have seen this as PABA on a bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA. Calculating Empirical Formulas

  4. 1) If you are given a percent composition, assume 100 grams. • Carbon: 61.31 g • Hydrogen: 5.14 g • Nitrogen: 10.21 g • Oxygen: 23.33 g Calculating Empirical Formulas

  5. 2) Use the molar mass to convert grams to moles. • Carbon • Hydrogen • Nitrogen • Oxygen Calculating Empirical Formulas

  6. 3) Calculate the mole ratio by dividing by the smallest number of moles: • Carbon • Hydrogen • Nitrogen • Oxygen Calculating Empirical Formulas

  7. 4) These are the subscripts for the empirical formula: • C7H7NO2 Calculating Empirical Formulas

  8. The empirical formula is not necessarily the actual, or molecular formula of the compound. • The molecular formula will be a multiple of the empirical formula. Molecular Formula from Empirical Formula

  9. Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C3H4. The experimentally determined molecular weight of this substance is 121 amu. What is the molecular formula of mesitylene? Molecular Formula from Empirical Formula

  10. Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C3H4. The experimentally determined molecular weight of this substance is 121 amu. What is the molecular formula of mesitylene? • C9H12 Molecular Formula from Empirical Formula

  11. Compounds containing C, H and O are routinely analyzed through combustion in a chamber like this. • C is determined from the mass of CO2 produced. • H is determined from the mass of H2O produced. • O is determined from the difference after the C and H have been determined. • Demo: Combustion of Glucose Combustion Analysis

  12. 1984 MC #73 – A 27.0-gram sample of an unknown hydrocarbon was burned in excess oxygen to form 88.0 grams of carbon dioxide and 27.0 grams of water. What is a possible molecular formula of the hydrocarbon? • A) CH4 • B) C2H2 • C) C4H3 • D) C4H6 • E) C4H10 Combustion Analysis

  13. 1984 MC #73 – A 27.0-gram sample of an unknown hydrocarbon was burned in excess oxygen to form 88.0 grams of carbon dioxide and 27.0 grams of water. What is a possible molecular formula of the hydrocarbon? • A) CH4 • B) C2H2 • C) C4H3 • D) C4H6 – medium difficulty, 44% correct • E) C4H10 Combustion Analysis

  14. 1989 MC #25 – The simplest formula for an oxide of nitrogen that is 36.8 percent nitrogen by weight is • A) N2O • B) NO • C) NO2 • D) N2O3 • E) N2O5 Practice AP Multiple Choice

  15. 1989 MC #25 – The simplest formula for an oxide of nitrogen that is 36.8 percent nitrogen by weight is • A) N2O • B) NO • C) NO2 • D) N2O3 – easy, 74% correct • E) N2O5 Practice AP Multiple Choice

  16. 1989 MC #39 – When a hydrate of Na2CO3 is heated until all of the water is removed, it loses 54.3% of its mass. The formula is • A) Na2CO3·10H2O • B) Na2CO3·7H2O • C) Not given in my archive… heh • D) Na2CO3·3H2O • E) Na2CO3·H2O Practice AP Multiple Choice

  17. 1989 MC #39 – When a hydrate of Na2CO3 is heated until all of the water is removed, it loses 54.3% of its mass. The formula is • A) Na2CO3·10H2O • B) Na2CO3·7H2O – medium, 51% correct • C) Not given in my archive… heh • D) Na2CO3·3H2O • E) Na2CO3·H2O Practice AP Multiple Choice

  18. 3.44, 3.46, 3.48, 3.52, 3.54 • Next week: • Tuesday: Reactions of Copper Lab Day 2 (Lab Cart will be available) – Oxide of Copper Lab Due • Wednesday: Balancing Equations (3.1) • Thursday: Predicting Products (3.2, 4.2-4.4) • Friday: Stoichiometry of Reactions (3.6) • Unit 2 Due Dates • 9/7: Oxide of Copper Lab Due • 9/15: Reactions of Copper Lab Due • 9/17: Problem Set 1 Due • 9/21: Basic Chemistry Skills Test Suggested Homework Problems

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