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 NH3(g) + 5 O2(g) 4 NO (g) + 6 H2O (g); A mixture, initially consisting of 7.2 torr of NH3(g) and 6.4 torr of O2(g), reacts at a certain temper-ature. At equilibrium, the partial pressure of H2O (g)is 2.4 torr. Calculate the partial pressure of O2(g)at equilibrium. *a) 4.4 torr b) 5.6 torr c) 3.4 torr d) 0.4 torr e) 1.6 torr 3224 NH(g) + 5 O(g) 4 NO (g) + 6 HO (g) 7.2 torr 6.4 torr 0 0 -4x -5x +4x initialchange22+6x 7.246.454x 6xWe are told ()2.4 6 2.4/ 60.4 ()6.456.45(0.4)6.42.04.4 finalxxP H OtorrxxtorrP Oxtorr2.What is the pH of a 3.2 M solution of H3PO4at 298K?( Ka1 = 7.2 x 10-3, Ka2 = 6.3 x 10-8, Ka3 = 4.2 x 10-13)a)pH = 3.35 b)pH = 1.07*c)pH = 0.82 d) pH = 1.41e)pH = 1.64 When one has a polyprotic weak acid and begins with the parent acid H2A or H3A, one does not have to worry about removal of the 2ndor 3rdproton (in rxns Ka2and Ka3). 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 H2SO4. 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. 3a134232412a224234KRxn:() ( ) () (), 7.2103.2KRxn:() ( ) () (), aH POaqH O lH OaqH POaqKxequilMxxxH POaqH O lH OaqHPOaq822313a3423436.310-KRxn:() ( ) () (), 4.210aaKxequilxyxyyHPOaqH O lH OaqPOaqKxequilyz22332413431133[()][()]7.210[()]3.23.2(3.2)(7.210)1.5210log([()])log(1.5210)0.82[check 5% rule: axyzzH OaqH POaqxxKxH POaqxxxxpHH OaqxH O 12662()]1.5210100(100)4.74%[()]3.2aqxxH C H Oaq
This preview has intentionally blurred sections.
Sign up to view the full version.