chem6bquiz#3

chem6bquiz#3 - Name_KEY Date Friday June 6 2008 Chem 6B...

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Name: ___________ KEY ___________ Date: Friday, June 6, 2008 Chem 6B, Spring 2008 TA: Angie Koo Quiz #3 40 points Show all work and calculations used to arrive at your answer. Adequate and CLEAR work with units and the correct amount of significant figures must be shown in order to receive ANY credit. Answers must be offset by placing a box around the final answer. 1) Find the Ka of an unknown monoprotic acid HX, given that a solution of 0.10M of an MgX 2 salt solution has a pH of 9.2. (12 points) MgX 3 Mg 2+ + 2X - ; [X - ] = 0.20M pOH = 14 – 9.2 = 4.8; [OH - ] = 10 -pOH = 10 -4.8 = 1.58 x 10 -5 M Equilibrium equation: X - + H 2 O HX + OH - I 0.20 0 0 -x +x +x e 0.20 – x x x Since we know that [OH - ] = 1.58 x 10 -5 M AT EQULIBRIUM (only equilibrium concentrations of OH- and H 3 O + determine pOH and pH, respectively), [OH - ] = x = [HX] = 1.58 x 10 -5 M K b = [HX][OH - ] [X - ] = (1.58 x 10 -5 ) 2 0.20- x = 1.25x10 -9 ; disregarding x works in this case with 5% rule BUT! You’re asked to get Ka, not Kb…
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chem6bquiz#3 - Name_KEY Date Friday June 6 2008 Chem 6B...

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