2009 Exam 1 Key

2009 Exam 1 Key - Name Section Number Chemistry 1314 Honors Midterm Exam#1 September 9 2009 This exam is closed book notes papers etc You may use a

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N a m e Section Number Chemistry 1314 Honors Midterm Exam #1 September 9, 2009 This exam is closed book, notes, papers, etc. You may use a calculator. If you need a physical constant or unit conversion factor that is not provided, please ask. This exam must represent your own work. Scores: 1. a. / 6 b. / 4 c. / 6 d. / 4 2. a. / 12 b. / 8 3. a. / 12 b. / 8 4. / 20 5. / 20 Total: / 100
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CHEM 1314 H Midterm Exam #1, 2009 Page 2 1. A gaseous element A forms two gaseous oxide compounds. The percent composition of each compound is found to be: Oxide % A (by mass) % oxygen (by mass) I 38.8 61.2 II 52.5 47.5 When 2.00 L of each oxide compound are decomposed into A and oxygen, the following volumes of gaseous A and oxygen are obtained: Oxide A (L) O 2 (L) I 2.00 7.00 II 1.00 2.00 a. By calculating mass ratios, show that these two compounds obey the law of multiple proportions. You do NOT need to use known atomic weights of the elements in your answer. The law of multiple proportions states that if two elements combine to form multiple compounds, the different masses of one element that combine with a fixed mass of the other element can be expressed as a ratio of small whole numbers. Taking an arbitrary fixed mass of oxygen (1.00 g), we need to determine the mass of A in the various compounds. () ( ) 7 : 4 75 . 1 : 1 105 . 1 : 634 . 0 : 105 . 1 5 . 47 5 . 52 00 . 1 : 634 . 0 2 . 61 8 . 38 00 . 1 : = = = = = II m I m A g O g A g O g II A g O g A g O g I A A The masses of A for a fixed amount of O in the two compounds are in a simple whole number ratio, in accordance with the law of multiple proportions. Fixing the mass of the oxygen (or A): 2 points Ratioing the masses of A (or oxygen): 2 points Demonstrating simple whole number ratio: 2 points
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CHEM 1314 H Midterm Exam #1, 2009 Page 3 b. Explain how these data reveal that element A must be atomic in nature. c. Demonstrate how these data reveal that gaseous element A is most reasonably assumed to be composed of diatomic molecules, A 2 . State any assumptions that you make. d. Determine the molecular formulae of compounds I and II, and determine the atomic mass of A. You may assume that oxygen’s atomic mass is 16.0 amu. Identify the element A. The masses of A that can combine with a fixed mass of O are in a simple whole number ratio, which therefore represent integer multiples of a fixed unit of mass, i.e. , a particle or “atom.” Assuming Avogadro’s hypothesis, two particles of compound II decomposes into one particle of element A. Since each II particle must contain equal amounts of A, then an A particle must contain an even number of A atoms to divide between the IIs. Two is the simplest choice of an even number, so until evidence requiring more than two A atoms in an
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This note was uploaded on 01/08/2012 for the course CHEM 1314 taught by Professor Gelder during the Fall '07 term at Oklahoma State.

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2009 Exam 1 Key - Name Section Number Chemistry 1314 Honors Midterm Exam#1 September 9 2009 This exam is closed book notes papers etc You may use a

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