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RelativeResourceManager 1 - CHEM 6B PRACTICE EXAM 3-1(Prof...

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Chem 6B Practice Exam 3-1 1 Prof. Crowell CHEM 6B PRACTICE EXAM 3-1 (Prof. Crowell) Spr 2010 May 17, 2010 *** FILL IN YOUR STUDENT ID NUMBER , NAME , and TEST FORM = A *** Bubble in the correct response on scantron (26 questions, worth 4 pts each) 1. Consider the following reaction: 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g); A mixture, initially consisting of 7.2 torr of NH 3 (g) and 6.4 torr of O 2 (g), reacts at a certain temper- ature. At equilibrium, the partial pressure of H 2 O (g) is 2.4 torr . Calculate the partial pressure of O 2 (g) at equilibrium. *a) 4.4 torr b) 5.6 torr c) 3.4 torr d) 0.4 torr e) 1.6 torr 3 2 2 4 NH (g) + 5 O (g) 4 NO (g) + 6 H O (g) 7.2 torr 6.4 torr 0 0 -4x -5x +4x initial change 2 2 +6x 7.2 4 6.4 5 4x 6x We are told ( ) 2.4 6 2.4/ 6 0.4 ( ) 6.4 5 6.4 5(0.4) 6.4 2.0 4.4 final x x P H O torr x x torr P O x torr 2. What is the pH of a 3.2 M solution of H 3 PO 4 at 298K? ( K a1 = 7.2 x 10 -3 , K a2 = 6.3 x 10 -8 , K a3 = 4.2 x 10 -13 ) a) pH = 3.35 b) pH = 1.07 *c) pH = 0.82 d) pH = 1.41 e) pH = 1.64 When one has a polyprotic weak acid and begins with the parent acid H 2 A or H 3 A, one does not have to worry about removal of the 2 nd or 3 rd proton (in rxns K a2 and K a3 ). The exception to this is if you are starting with an intermediate in the reaction, or if you are starting with a strong acid such as H 2 SO 4 . The reason we do not need to worry about rxns 2 and 3 is that production of a proton suppresses the subsequent reactions, since this is one of the products of reaction 1. 3 a1 3 4 2 3 2 4 1 2 a2 2 4 2 3 4 K Rxn: ( ) ( ) ( ) ( ), 7.2 10 3.2 K Rxn: ( ) ( ) ( ) ( ), a H PO aq H O l H O aq H PO aq K x equil M x x x H PO aq H O l H O aq HPO aq 8 2 2 3 13 a3 4 2 3 4 3 6.3 10 - K Rxn: ( ) ( ) ( ) ( ), 4.2 10 a a K x equil x y x y y HPO aq H O l H O aq PO aq K x equil y z 2 2 3 3 2 4 1 3 4 3 1 1 3 3 [ ( )][ ( )] 7.2 10 [ ( )] 3.2 3.2 (3.2)(7.2 10 ) 1.52 10 log([ ( )]) log(1.52 10 ) 0.82 [ check 5% rule: a x y z z H O aq H PO aq x x K x H PO aq x x x x pH H O aq x H O     1 2 6 6 2 ( )] 1.52 10 100 (100) 4.74% [ ( )] 3.2 aq x x H C H O aq
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