Lesson 10.3 Percent Composition and Chemical Formulas
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1 10.3 Lesson 10.3 Percent Composition and Chemical Formulas Objectives Lesson Links Calculate the percent composition of a compound Calculate the empirical formula of a compound Distinguish between empirical and molecular formulas Ch. 10 Small Scale Lab 13: Measuring Mass: A Means of Counting Ch. 10 Core TR: Section 3 Review 10.3 Chemistry Tutorial: Calculating Percent Composition from Mass and from a Formula 10.3 Chemistry Tutorial: Determining the Empirical Formula of a Compound 10.3 Lesson Overview (PowerPoint file) Chapter 10 Online Student Edition Ch. 10 Concepts in Action: Your Daily Percent Composition Study WB Chapter 10 Lesson 3 Overview/Materials Overview In this lesson you will cover the calculations of percent composition and empirical formula of a compound. You will also compare the molecular formula of a compound to the compound's empirical formula. Classroom Materials Standard 2 Block 1 Quick Lab: 3 medium-sized test tubes, balance, spatula, hydrated compounds of copper(ii) sulfate, calcium chloride, and sodium sulfate, test tube holder, gas burner Empirical Formulas: 3 red marbles, 6 green marbles, 3 black marbles, and 12 blue marbles Standard page 1 of 8
2 There are no items. Chemistry & You Engage Have students study the photograph that accompanies the Chemistry & You feature on p Chapter 10 Online Student Edition Ask What should be the total percent of materials listed on the label? (100%) Ask If the shirt were made of 25% polyester, instead of 15%, what percent of the shirt would be made of cotton? (75%) Activate Prior Knowledge Engage Review the law of definite proportions and the concept of lowest whole-number ratios. Tell students to keep these concepts in mind as they proceed with the lesson. Percent Composition of a Compound Explain Figure 10.9_part1 Have students study the circle graphs in Figure 10.9_part1 and Figure 10.9_part2, and read the text on percent composition on p Point out that the percentages given in the three sectors add up to a total of 100%. Figure 10.9_part2 Chapter 10 Online Student Edition Ch. 10 Math Tutorial B: Circle Graphs Ask Which compound is a better source of potassium? Why? (K2CrO4, because it has a greater percentage of potassium than K2Cr2O7) Have students complete the Ch. 10 Math Tutorial B: Circle Graphs on PearsonChem.com. page 2 of 8
3 Percent Composition of a Compound Explain Percents are used to show relative parts of mixtures as well as the composition of a compound. But when an extremely small amount of a substance is present in a large amount of another substance, it might not be practical to use percents (parts per one hundred) to show the makeup of the mixture. Instead, concentrations of extremely dilute solutions are sometimes measuring in units of parts per million ( ppm) or parts per billion (ppb). For example, the composition of a mixture that consists of 1 gram of a substance per 10 6 grams of water (or 1 milligram of substance per liter of water) can be expressed as 1 ppm. Ch. 10 Concepts in Action: Your Daily Percent Composition Have students complete the Ch. 10 Concepts in Action: Your Daily Percent Composition on PearsonChem.com. Percent Composition of a Compound Extend Challenge students to develop a spreadsheet that can calculate the percent composition of a chemical compound. Tell students they should consider how to manage the initial inputs, such as the chemical formula and the molar masses of each element in the formula, as well as the formulas needed to complete their final calculations. Encourage students to demonstrate and explain their spreadsheets to interested students. Quick Lab Purpose After completing this activity, students will be able to determine the percent of water in a hydrate. Materials: 3 medium-sized test tubes, balance, spatula, hydrated compounds of copper(ii) sulfate, calcium chloride, and sodium sulfate, test tube holder, gas burner page 3 of 8
4 Prep Time 20 minutes Class Time 30 minutes Safety Students should wear safety goggles and tie back loose hair. Caution students that while heating test tubes, they should not aim the opening of the tube toward anyone. Tell them to move the test tube in the flame and not to heat one spot excessively. CAUTION! Be sure that students allow the tubes to cool completely before they touch them. Hot glass looks exactly like cold glass! Expected Outcome See the Sample Data below for CuSO4 5H2O/ CaCl2 H2O/ Na2SO4 10H2O Test tube + hydrate (before heating): 23.88g/ 23.60g/ 23.92g Empty test tube: 21.19g/ 21.25g/ 21.17g Mass of hydrate: 2.69g/ 2.35g/ 2. 75g Test tube + salt (after heating): 22.88g/ 23.07g/ 22.71g Empty test tube: 21.19g/ 21.25g/ 21.17g Mass of anhydrous salt: 1.69g/ 1. 82g/ 1.54g Mass of water lost: 1.00g/ 0.53g / 1.21g Percent water (experimental): 3 7.2%/ 22.6%/ 44.0% Percent water (theoretical): 36. 1%/ 24.5%/ 55.9% For Enrichment Have students design and conduct a similar experiment to determine the percent of oxygen in potassium chlorate. Tell students that when potassium chlorate is heated, potassium chloride and oxygen are produced: 2KClO3 2KCl + 3O2. For classroom safety, no more than 5 g of potassium chlorate should be used. Results should show that potassium chlorate is approximately 39% oxygen. Empirical Formulas Explore page 4 of 8
5 Use a class activity to provide students with an analogy that helps clarify the concepts of percent composition and empirical formulas. Provide pairs of students with sets of marbles. Have students express the number of different colored marbles as fractions and percents of the whole collection. Ask What percent of the collection do the red marbles represent? (12.5%) Show them that the sums of the fractions and percents are equal to 1 and 100% respectively. Ask What is the ratio of red:green:black: blue marbles in lowest terms? (1:2:1:4) This activity can be extended if the different colored marbles are assumed to be atoms of different elements. Ask What is the empirical formula of a hypothetical "compound" that consists of 2 5% red marbles and 75% green marbles? ( The ratio of red marbles to green marbles in the empirical formula would be 1:3.) Students should express percent composition of the marbles and determine the "empirical formula" of a marble combination. Materials: 3 red marbles, 6 green marbles, 3 black marbles, and 12 blue marbles Empirical Formulas Explain Direct students to examine Sample Problem on page 331. Explain that when using percent composition to determine an empirical formula, g of compound is often arbitrarily chosen because it is easy to use. If an element comprises 28.5% of the mass of a compound, for example, it makes up 28.5 g of a g sample. Any other mass of compound can be used but computation will be more difficult Chemistry Tutorial: Determining the Empirical Formula of a Compound Have them complete 10.3 Chemistry Tutorial: Determining the Empirical Formula of a Compound on PearsonChem.com for extra practice. page 5 of 8
6 Molecular Formulas Explain Table 10.3 Have students examine the patterns shown in Table Then have them add entries for these pairs of compounds: nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4), diphosphorus pentoxide (P2O5) and tetraphosphorus decoxide (P4O10). Instruct students to determine the empirical formula for each pair. Molecular Formulas Extend Tell students that a variety of carbohydrates have the empirical formula CH2O. Two such carbohydrates are glucose, which is abundant in plants and animals, and fructose, which is found in fruits and honey. Point out that glucose and fructose both share the same molecular formula: C6H12O6. Ask Do you think these carbohydrates have the same structure? Why? (no, because they have different names) Ask Do you think there are other carbohydrates with this molecular formula? ( Answers will vary.) Challenge students to research and draw the structures of glucose and fructose, and list and additional carbohydrates that share the chemical formula C6H12O6. Tell students they can learn more about these types of monosaccharides in Chapter 24. Assess and Remediate Evaluate Have students list verbally or in writing the steps they would take to calculate the page 6 of 8
7 molecular formula in each of the following situations: 1. The empirical formula and molar mass are known. 2. The percent composition and molar mass are known. Then have students complete the 10.3 Lesson Check. Remediate Point out to students that when they know the percent composition and molar mass of a compound they must first use the percent composition to calculate the empirical formula. They can then calculate the empirical formula mass and compare it to the molar mass of the molecular compound to determine the molecular formula. Help students to construct a flowchart or a concept map depicting the various scenarios. Chapter 10 Review Choose from the chapter review and assessment resources for Chapter 10: Chemical Quantities listed at the right. Ch. 10 Math Tutorial A: Unit Conversion Ch. 10 Math Tutorial B: Circle Graphs Ch. 10 Core TR: Practice Problems Ch. 10 Core TR: Interpreting Graphics Ch. 10 Core TR: Vocabulary Review Ch. 10 Core TR: Chapter Quiz Ch. 10 Core TR: Chapter Test A Ch. 10 Core TR: Chapter Test B Differentiated Instruction Less Proficient Readers In small groups, have students develop numbered lists of steps to follow for determining empirical formulas from percent composition, determining empirical formulas Study WB Chapter 10 Lesson 3 Ch. 10 Core TR: Section 3 Review page 7 of 8
8 from mass data, and determining molecular formulas from the empirical formula and molar mass. Special Needs Students For the class activity involving the colored marbles, consider replacing the marbles with larger tactile colored objects, larger tactile textured objects, or colored pieces of candy. Issues involving potential swallowing of the materials or sign impairment should be considered when selecting materials for the lab. Advanced Students Have students research the formulas of the three different oxides of iron. Ask Which of the oxides contains the highest percent of iron? (Of FeO (77.7% Fe), Fe2O3 (69.9% Fe), and Fe3O4 (72.3% Fe), FeO has the highest percentage of iron.) Focus on ELL Frontload the Lesson As a class, determine the percentage of students in the room who are freshmen, sophomores, juniors, and seniors. Remind students that a percentage is determined by writing the part over the total and then multiplying by 100. Explain that they have calculated the percent composition of the class. Ask students to use what they learned from this activity to predict how the percent composition of a chemical compound is calculated. Ch. 10 Core TR: Section 3 Review Study WB Chapter 10 Lesson 3 My Notes Homework page 8 of 8
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