2/4/14. What does the percent composition of a compound tell you?

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1 103 Percent Composition 103 Percent Composition CHEMISTRY & YOU Chapter 10 Chemical Quantities 101 The Mole: A Measurement of Matter 10 Mole-Mass and Mole-Volume Relationships 103 Percent Composition and Chemical Formulas What does the percent composition of a compound tell you? A tag sewn into the seam of a shirt usually tells you what fibers were used to make the cloth and the percent of each 1 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition How do you calculate the percent composition of a compound? 103 Percent Composition The relative amounts of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound 3 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 4 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 1

2 103 Percent Composition 103 Percent Composition The percent composition of potassium chromate, K CrO 4, is: These percents must total 100% K = 403% Cr = 68% O = 39% K = 403% Cr = 68% + O = 39% 100% 5 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 6 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition These percents must total 100% The percent composition of a compound is always the same 103 Percent Composition Percent Composition from Mass Data If you know the relative masses of each element in a compound, you can calculate the percent composition of the compound 7 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 8 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved

3 103 Percent Composition 103 Percent Composition Sample Problem 109 Percent Composition from Mass Data The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100% % by mass of element = mass of element mass of compound Calculating Percent Composition from Mass Data When a 1360-g sample of a compound containing only magnesium and oxygen is decomposed, 540 g of oxygen is obtained What is the percent composition of this compound? 9 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 10 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition Sample Problem Percent Composition Sample Problem Analyze List the knowns and the unknowns The percent by mass of an element in a compound is the mass of that element divided by the mass of the compound multiplied by 100% KNOWNS mass of compound = 1360 g mass of oxygen = 540 g O mass of magnesium = 1360 g 540 g O = 80 g Mg UNKNOWNS percent by mass of Mg =?% Mg percent by mass of O =?% O Calculate Solve for the unknowns Determine the percent by mass of Mg in the compound % Mg = mass of Mg mass of compound = 80 g 1360 g = 603% Mg 11 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 1 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 3

4 103 Percent Composition Sample Problem Percent Composition Sample Problem 109 Calculate Solve for the unknowns Determine the percent by mass of O in the compound % O = mass of O mass of compound 3 Evaluate Does the result make sense? The percents of the elements add up to 100% 603% + 397% = 100% 540 g = 1360 g = 397% O 13 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 14 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition 15 Percent Composition from the Chemical Formula You can also calculate the percent composition of a compound using its chemical formula The subscripts in the formula are used to calculate the mass of each element in a mole of that compound Using the individual masses of the elements and the molar mass, you can calculate the percent by mass of each element Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition 16 Percent Composition from the Chemical Formula You can also calculate the percent composition of a compound using its chemical formula % by mass of element mass of element in 1 mol compound = molar mass of compound Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 4

5 103 Percent Composition Sample Problem Percent Composition Sample Problem Calculating Percent Composition from a Formula Propane (C 3 H 8 ), the fuel commonly used in gas grills, is one of the compounds obtained from petroleum Calculate the percent composition of propane Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 18 1 Analyze List the knowns and the unknowns Calculate the percent by mass of each element by dividing the mass of that element in one mole of the compound by the molar mass of the compound and multiplying by 100% KNOWNS mass of C in 1 mol C 3 H 8 = 3 mol 10 g/mol = 360 g mass of H in 1 mol C 3 H 8 = 8 mol 10 g/mol = 80 g molar mass of C 3 H 8 = 360 g/mol + 80 g/mol = 440 g/mol UNKNOWNS percent by mass of C =?% C percent by mass of H =?% H Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition Sample Problem Percent Composition Sample Problem 1010 Calculate Solve for the unknowns Determine the percent by mass of C in C 3 H 8 % C = mass of C in 1 mol C 3 H 8 molar mass of C 3 H g = 440 g Calculate Solve for the unknowns Determine the percent by mass of H in C 3 H 8 % H = mass of H in 1 mol C 3 H 8 molar mass of C 3 H 8 80 g = 440 g = 818% C = 18% H 19 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 0 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 5

6 103 Percent Composition 3 The percents of the elements add up to 100% when the answers are expressed to two significant figures (8% + 18% = 100%) Sample Problem 1010 Evaluate Does the result make sense? 103 Percent Composition Percent Composition as a Conversion Factor You can use percent composition to calculate the number of grams of any element in a specific mass of a compound To do this, multiply the mass of the compound by a conversion factor based on the percent composition of the element in the compound 1 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition Propane is 818% carbon and 18% hydrogen Percent Composition as a Conversion Factor You can use the following conversion factors to solve for the mass of carbon or hydrogen contained in a specific amount of propane 103 Percent Composition CHEMISTRY & YOU What information can you get from the percent composition of a compound? 818 g C and 18 g H 3 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 4 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 6

7 103 Percent Composition CHEMISTRY & YOU 103 Percent Composition Sample Problem 1011 What information can you get from the percent composition of a compound? You can use percent composition to determine the mass of an element in a sample of a compound of a given size You can also determine the empirical formula of the compound Calculating the Mass of an Element in a Using Percent Composition Calculate the mass of carbon and the mass of hydrogen in 80 g of propane (C 3 H 8 ) 5 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 6 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 103 Percent Composition Sample Problem Percent Composition Sample Problem Analyze List the known and the unknowns Use the conversion factors based on the percent composition of propane to make the following conversions: grams C 3 H 8 grams C and grams C 3 H 8 grams H KNOWN mass of C 3 H 8 = 80 g UNKNOWNS mass of carbon =? g C mass of hydrogen =? g H Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 8 Calculate Solve for the unknowns To calculate the mass of C, first write the conversion factor to convert from mass of C 3 H 8 to mass of C From Sample 818 g C Problem 1010, the 100 g C 3 H percent by mass of C 8 in C 3 H 8 is 818% Multiply the mass of C 3 H 8 by the conversion factor 818 g C 80 g C 3 H 8 = 671 g C Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 7

8 103 Percent Composition Sample Problem Percent Composition Sample Problem 1011 Calculate Solve for the unknowns To calculate the mass of H, first write the conversion factor to convert from mass of C 3 H 8 to mass of H From Sample 18 g H Problem 1010, the percent by mass of H in C 3 H 8 is 18% 3 Evaluate Does the result make sense? The sum of the two masses equals 8 g, the sample size, to two significant figures (67 g C + 15 g H = 8 g C 3 H 8 ) 9 Multiply the mass of C 3 H 8 by the conversion factor 18 g H 80 g C 3 H 8 = 15 g H Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 30 Copyright Pearson Education, Inc, or its affiliates All Rights Reserved 8

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