CHM1045 Lecture 8 Outline of Last Lecture I Atomic Mass 1 Example 1 II Avogrado s Number and the Molar Mass 1 Example 2 2 Example 3 3 Example 4 III Molecular Mass 1 Example 5 2 Example 6 3 Example 7 Outline of Current Lecture I Percent Composition of Compounds 1 Example 1 2 Example 2 II Percent composition Empirical Formulas 1 Example 3 2 Example 4 3 Example 5 4 Example 6 Current Lecture Percent Composition of an element in a compound n x molar mass of element Molar mass of compound N of moles f the element in 1 mole of the compound Example 1 C2H6 O C 2 x 12 01 g 46 07 g H 6 x 1 008g 46 07 X 100 52 14 X 100 13 13 O 1 x 16 0g 46 07 X 100 34 73 52 14 13 13 34 73 100 00 Example 2 Phosphoric acid H3PO4 is a colorless syrupy liquid used in detergents fertilizers toothpastes and in carbonated beverages for a tangy flavor Calculate the percent composition be mass H P and O in this compound H3PO4 3X1 008 30 97 4x16 00 97 99g mol H 3 x 1 008 X 100 3 09 97 99 P 30 97 X 100 31 6 97 99 O 4x16 X 100 65 3 97 99 3 09 31 6 65 3 99 99 Percent composition and Empirical Formulas Mass Convert to grams by molar mass Moles of each element by smallest of moles Mole ratios by elements change to integer subscripts Empirical Formula Example 3 Ascorbic acid Vitamin C cures scurvy It is composed of 40 92 percent carbon C 4 58 percent hydrogen H and 54 50 percent oxygen O by mass Determine its empirical formula Assume 100g of ascorbic acid Step 1 C 40 92 g X 1 mol C 3 41 Mol C 12 01g C H 4 58 g X 1 mol H 4 54 Mol H 1 008 g H O 54 54 g X 1mol O 3 41 Mol O 16 00 gO Step 2 C H O 3 41 3 41 1 4 54 3 41 1 33 3 41 3 41 1 C1H1 33O1 We need to convert 1 33 the subscript for H into an integer This can be done by trial and error 1 33 1 1 33 1 33 2 2 66 1 33 3 3 99 4 Because 1 33 3 gives us an integer 4 we multiply all the subscripts by 3 and you ll get C3H4O3 Example 4 Chalcopyrite CuFeS2 is a principal mineral of copper Calculate the number of kilograms of Cu in 3 71 103 kg of chalcopyrite CuFeS2 Cu 63 55 g mol Fe 55 85 g mol S2 2 32 07 g mol 183 54 g mol Cu 63 55 X 100 34 62 Cu 183 54 3 71 103 kg 0 3462 1 28 103 kg Cu Example 5 Empirical Formula of ethanol Combust 11 5 g Ethanol Collect 22g CO 2 13 5g H2O Step 1 22 0 g CO2 1mol CO2 1molC 12 01gC 6g C 44 01 g CO2 1 mol CO2 1 mole C 13 5 g H2O 1 mol H20 18 02g H20 Step 2 11 5 g Ethanol 6 09 g C 2 mol H 1 mol H2O 1 008 H 1 51g H 1 mol H 1 51g H 3 99 Oxygen Step 3 6 00 g C 1 mol 12 01 g C 5 mol C 1 51 g H 1 mol H 1 50 mol H 1 008 g H 3 99 g O 1 mol O 25 Mol O 16 0 g O Step 4 Divide by the smallest number C 5 2 H 1 5 6 O 25 1 25 25 25 C2H6O Example 6 A sample of a compound contains 30 46 percent nitrogen and 69 54 percent oxygen by mass as determined by a mass spectrometer In a separate experiment the molar mass of the compound is found to be between 90g and 95g Determine the molecular formula and the accurate molar mass of the compound Assume 100g 30 46 30 46 g N 69 54 69 54 g O 30 46 g N 1 mol N 2 18 Mol N 14 00 g N 69 54 g O 1 mol O 4 35 Mol O 16 0 g O N2 18 O 4 35 N 2 18 1 2 18 O 4 35 2 2 18 NO2 Empirical Formula 14 00 2 16 46 00 g mol molar mass empirical molar mass 90 2 46 95 2 41 2 NO2 N2O4 Molecular Formula
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