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classes_fall07_20A-1ID12_MicrosoftWord-20A-1_practice_final_SOLUTIONS

# Classes_fall07_20A-1ID12_MicrosoftWord-20A-1_practice_final_SOLUTIONS

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1 PRACTICE FINAL EXAM: CHEM 20A-1, 2007: SOLUTIONS 1. A mixture of iron (Fe) and aluminum (Al) weighs 9.62 g. The sample is reacted with an excess of HCl and produces 0.738 g of H 2 according to the reactions 2 Al + 6 HCl = 2 AlCl 3 + 3 H 2 Fe + 2 HCl = FeCl 2 + H 2 How much iron was present in the original mixture? Let x be the number of grams of Fe in the original mixture; then (9.62-x) is the number of grams of Al originally present. For every mole of Al that reacts, 3/2 mole of H 2 are produced; and for every mole of Fe that reacts, 1 mole of H 2 is produced. The total number of moles of H 2 produced is 0.738 g H 2 2.02 g H 2 / mole H 2 = 0.365 mole H 2 , from which it follows that 3 2 (9.62 - x ) 27.0 + x 55.8 = 0.365 . (The atomic weights of Al and Fe are 27.0 and 55.8.) Solving for x gives x = 4.50 g. 2. Consider the ionic diatomic LM whose dipole moment is 8.0 debye (D) and whose bond length is 2.02A. What is the percent ionic character of this bond? 1 D = 3.336 x 10 -30 C-m, implying 8.0 D = 2.67 x 10 -29 C-m. The relationship between dipole moment ( μ ) and bond length (R) is μ = qR, where q is the magnitude of the charge that is separated, i.e., charges of +q and –q are separated by a distance R. Solving for q we have q = μ /R = (2.67 x 10 -29 C-m)/(2.02 x 10 -10 m) = 1.32 x 10 -19 C. Thus the fraction of a fundamental charge (e = 1.60 x 10 -19 C) that is separated is (1.32 x 10 -19 C-m)/(1.60 x 10 -19 m) = 0.826, implying a percent ionic character of 82.6. 3. Consider two hydrogen nuclei (protons), separated by 1A along the z-axis; suppose one is at the origin, with coordinates (x, y, z) = (0, 0, 0), and the other is at the point (0, 0, 1A). a.

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Classes_fall07_20A-1ID12_MicrosoftWord-20A-1_practice_final_SOLUTIONS

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