10.3 Percent Composition and Chemical Formulas

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1 Name Class Date 10.3 Percent Composition and Chemical Formulas Essential Understanding of its empirical formula. A molecular formula of a compound is a whole-number multiple Lesson Summary Percent Composition of a Compound Percent composition is the percent by mass of each element in a compound. To find the percent by mass of an element in a compound, use the formula: mass of element % mass of element = 100 % mass of compound To find the mass of an element in a sample of a compound, use the formula: mass of element in 1 mol compound % mass = 100 % molar mass of compound Empirical Formulas The empirical formula of a compound is the formula with the smallest whole-number mole ratio of the elements. An empirical formula may or may not be the same as the actual molecular formula. Molecular Formulas A molecular formula specifies the actual number of atoms in each element in one molecule or formula unit of the substance. To find a molecular formula, the molar mass of the compound must be determined. After reading Lesson 10.3, answer the following questions. Percent Composition of a Compound 1. How do you express relative amounts of each element in a compound? 2. Circle the letter of the phrase that completes this sentence correctly. The number of percent values in the percent composition of a compound is a. half as many as there are different elements in the compound. b. as many as there are different elements in the compound. c. twice as many as there are different elements in the compound. 3. What is the formula for the percent by mass of an element in a compound? Workbook 10.3 page 1

2 Name Class Date 4. In the diagram below, which compound has a greater percent composition of chromium? How many more percentage points is this? K 2CrO 4 K 2Cr 2O 7 Potassium chromate, K2CrO4 Potassium dichromate, K2Cr2O7 5. To calculate the percent composition of a known compound, start with the chemical formula of the compound and calculate the, which gives the mass of one mole of the compound. 6. Is the following sentence true or false? You can use percent composition to calculate the number of grams of an element in a given amount of a compound. 7. How do you calculate the grams of an element in a specific amount of a compound? Empirical Formulas 8. An empirical formula of a compound gives the whole-number ratio of the atoms of the elements in a compound. 9. Is the following sentence true or false? The empirical formula of a compound is always the same as the molecular formula. 10. Look at Figure and Table Name three compounds that have an empirical formula of CH. Workbook 10.3 page Fill in the labels on the diagram below.

3 SO3 molecule composed of S atom and 3 atoms MICROSCOPIC INTERPRETATION SO 3 MACROSCOPIC INTERPRETATION sulfur atoms 1 mol SO3 composed of and ( ) oxygen atoms Molecular Formulas 12. The molecular formula of a compound is either the same as its empirical formula or a of it. 13. What do you need to know to calculate the molecular formula of a compound? 14. If you divide the molar mass of a compound by the empirical formula mass, what is the result? 15. What factor would you use to convert the empirical formula of a compound to a molecular formula? Workbook 10.3 page3

4 Name Date Class PERCENT COMPOSITION AND CHEMICAL FORMULAS Section Review Objectives Calculate the percent by mass of an element in a compound Interpret an empirical formula Compare and contrast empirical and molecular formulas Vocabulary percent composition empirical formula Key Equation mass of element % mass of element = 100% mass of compound Part A Completion Use this completion exercise to check your knowledge of the terms and your understanding of the concepts introduced in this section. Each blank can be completed with a term, short phrase, or number. The 1 of a compound is the percent by mass of each element in a compound. The percent by mass of an element in a compound is the number of grams of the element per 2 g of the compound, multiplied by 100%. To calculate the percent by mass of an element in a known compound, divide the mass of the element in one mole by the 3 and multiply by 100%. A(n) 4 formula represents the lowest 5 ratio of the elements in a compound. It can be calculated from a compound s percent composition. The 6 formula of a compound is either the same as its empirical formula, or it is some whole-number multiple of it Chapter 10 Chemical Quantities section review 1

5 Name Date Class Part B True-False Classify each of these statements as always true, AT; sometimes true, ST; or never true, NT. 7. It is necessary to know the formula of a compound in order to calculate its percent composition. 8. If the percent by mass of carbon in methane, CH 4, is 75%, then 100 grams of methane contain 25.0 grams of hydrogen. 9. The formula for methane, CH 4, is both a molecular and an empirical formula. 10. The empirical formula for glucose, C 6H 12O 6, is C 2H 4O 2. Part C Matching Match each description in Column B to the correct term in Column A. Column A 11. percent composition 12. empirical formula 13. molecular formula Column B a. describes the actual number of atoms of each element in a molecule of a compound b. the lowest whole-number ratio of atoms of the elements in a compound c. the percent by mass of each element in a compound

6 Part D Problems Solve the following problems in the space provided. Show your work. 14. What is the percent composition of each of the following? a. Cr 2O 3 c. HgS b. Mn 2P 2O 7 d. Ca(NO 3) Determine the empirical formula of the compound with the percent composition of 29.1% Na, 40.5% S, and 30.4% O. 16. How many kilograms of iron can be recovered from 639 kilograms of the ore Fe 2O 3? 2 Chapter 10 Chemical Quantities section review

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