# solut03 - 9 CHAPTER THREE STOICHIOMETRY Atomic Masses and...

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Unformatted text preview: 9 CHAPTER THREE STOICHIOMETRY Atomic Masses and the Mass Spectrometer 21. A = atomic mass = 0.7899(23.9850 amu) + 0.1000(24.9858 amu) + 0.1101(25.9826 amu) A = 18.95 amu + 2.499 amu + 2.861 amu = 24.31 amu 22. Let x = % of 151 Eu and y = % of 153 Eu, then x + y = 100 and y = 100 - x . 151.96 = 100 209) x)(152.9- (100 + ) x(150.9196 15196 = 150.9196 x + 15292.09 - 152.9209 x , -96 = -2.0013 x x = 48%; 48% 151 Eu and 100 - 48 = 52% 153 Eu 23. 186.207 = 0.6260(186.956) + 0.3740(A), 186.207 - 117.0 = 0.3740(A) A = 0.3740 69.2 = 185 amu (A = 184.95 amu without rounding to proper significant figures.) 24. A = 0.2100(283.9 amu) + 0.3154(284.8 amu) + 0.4746(287.8 amu) = 286.0 amu 25. A = 0.0140(203.973) + 0.2410(205.9745) + 0.2210(206.9759) + 0.5240(207.9766) A = 2.86 + 49.64 + 45.74 + 109.0 = 207.2 amu; From the periodic table, the element is Pb. 26. There are three peaks in the mass spectrum, each 2 mass units apart. This is consistent with two isotopes, differing in mass by two mass units. The peak at 157.84 corresponds to a Br 2 molecule composed of two atoms of the lighter isotope. This isotope has mass equal to 157.84/2 or 78.92. This corresponds to 79 Br. The second isotope is 81 Br with mass equal to 161.84/2 = 80.92. The peaks in the mass spectrum correspond to 79 Br 2 , 79 Br 81 Br and 81 Br 2 in order of increasing mass. The intensities of the highest and lowest mass tell us the two isotopes are present at about equal 10 abundance. The actual abundance is 50.68% 79 Br and 49.32% 81 Br. CHAPTER 3 STOICHIOMETRY 11 27. GaAs can be either 69 GaAs or 71 GaAs. The mass spectrum for GaAs will have 2 peaks at 144 (= 69 + 75) and 146 (= 71 + 75) with intensities in the ratio of 60:40 or 3:2. Ga 2 As 2 can be 69 Ga 2 As 2 , 69 Ga 71 GaAs 2 or 71 Ga 2 As 2 . The mass spectrum will have 3 peaks at 288, 290, and 292 with intensities in the ratio of 36:48:16 or 9:12:4. We get this ratio from the following probability table: 69 Ga (0.60) 71 Ga (0.40) 69 Ga (0.60) 0.36 0.24 71 Ga (0.40) 0.24 0.16 Moles and Molar Masses 28. a. 3.00 × 10 20 molecules HF × HF mol HF g 20.01 _ molecules 10 _ 6.022 HF mol 1 23 = 9.97 × 10-3 g HF b. 3.00 × 10-3 mol HF × HF mol HF g 20.01 = 6.00 × 10-2 g HF c. 1.5 × 10 2 mol HF × HF mol HF g 20.01 = 3.0 × 10 3 g HF 146 144 288 290 292 12 CHAPTER 3 STOICHIOMETRY 12 d. 1 molecule HF × HF mol HF g 20.01 _ HF molecules 10 _ 6.022 HF mol 1 23 = 3.323 × 10-23 g HF e. 18.0 pmol HF × HF mol HF g 20.01 _ pmol 10 HF mol 1 12 = 3.60 × 10-10 g HF = 360. pg HF CHAPTER 3 STOICHIOMETRY 13 29. a. 100 molecules H 2 O × O H molecules 10 _ 6.022 O H mol 1 2 23 2 = 1.661 × 10-22 mol H 2 O b. 100.0 g H 2 O × O H g 18.015 O H mol 1 2 2 = 5.551 mol H 2 O c. 500 atoms Fe × atoms 10 _ 6.022 Fe mol 1 23 = 8.303 × 10-22 mol Fe d. 500.0 g Fe × Fe g 55.85 Fe mol 1 = 8.953 mol Fe e. 150 molecules N 2 × N molecules 10 _ 6.022 N mol 1 2 23 2 = 2.491 × 10-22 mol N 2 f. 150.0 g Fe 2 O 3 × g 159.70 mol 1 = 0.9393 mol Fe 2 O 3 g. 10.0 mg NO 2 × g...
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solut03 - 9 CHAPTER THREE STOICHIOMETRY Atomic Masses and...

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