H06 Buffer Solution - patino (mp25752) H06: Buffers McCord...

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Unformatted text preview: patino (mp25752) H06: Buffers McCord (53215) 1 This print-out should have 21 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. You might want to wait on some of these questions until you have heard me lecture on the subject. Youve been warned.- Dr. McCord 001 10.0 points Calculate the concentration of HCO 3 present in 0 . 3274 M H 2 CO 3 . Correct answer: 0 . 000375209 mol / L. Explanation: K w = 1 10 14 The equilibrium reactions of interest are H 2 CO 3 (aq) + H 2 O( ) H 3 O + (aq) + HCO 3 (aq) K a1 = 4 . 3 10 7 HCO 3 (aq) + H 2 O( ) H 3 O + (aq) + CO 2 3 (aq) K a2 = 5 . 6 10 11 Because the second ionization constant is much smaller than the first, we can assume that the first step dominates. H 2 CO 3 + H 2 O H 3 O + + HCO 3 . 3274 x + x + x . 3274 x x x Assuming that x . 3274 M, K a1 = [H 3 O + ][HCO 3 ] [H 2 CO 3 ] 4 . 3 10 7 = x 2 . 3274 x x 2 . 3274 x = radicalBig (0 . 3274) (4 . 3 10 7 ) = 0 . 000375209 M . Because x < 1% of 0 . 3274 M, the assump- tion was valid, so x = [H 3 O + ] = [HCO 3 ] = 0 . 000375209 M . 002 10.0 points Scientists investigating acid rain measured the pH of a water sample from a lake and found it to be 4 . 8. The total concentra- tion of dissolved carbonates in the lake is . 0045 M. Determine the molar concentra- tion of H 2 CO 3 in the lake if K a1 = 4 . 3 10 7 and K a2 = 5 . 6 10 11 . Correct answer: 0 . 00438113 mol / L. Explanation: pH = 4 . 8 K a1 = 4 . 3 10 7 K a2 = 5 . 6 10 11 C carbonates = 0 . 0045 M [H 3 O + ] = 10 pH = 10 4 . 8 = 1 . 58489 10 5 The total concentration is . 0045 = [CO 2 3 ] + [HCO 3 ] + [H 2 CO 3 ] (1) The first equilibrium reaction is H 2 CO 3 (aq) + H 2 O( ) HCO 3 (aq) + H 3 O + K a1 = 4 . 3 10 7 K a1 = [H 3 O + ][HCO 3 ] [H 2 CO 3 ] 4 . 3 10 7 = (1 . 58489 10 5 ) [HCO 3 ] [H 2 CO 3 ] [HCO 3 ] = (4 . 3 10 7 ) [H 2 CO 3 ] 1 . 58489 10 5 = 0 . 0271312[H 2 CO 3 ] (2) The second equilibrium reaction is HCO 3 (aq) + H 2 O( ) CO 2 3 (aq) + H 3 O + K a2 = 5 . 6 10 11 K a2 = [H 3 O + ][CO 2 3 ] [HCO 3 ] 5 . 6 10 11 = (1 . 58489 10 5 ) [CO 2 3 ] [HCO 3 ] [HCO 3 ] = (1 . 58489 10 5 ) [CO 2 3 ] 5 . 6 10 11 = 2 . 83017 10 5 [CO 2 3 ] (3) Equating (2) and (3), . 0271312[H 2 CO 3 ] = 2 . 83017 10 5 [CO 2 3 ] patino (mp25752) H06: Buffers McCord (53215) 2 [H 2 CO 3 ] = (1 . 04314 10 7 ) [CO 2 3 ] (4) Substituting equations (3) and (4) into equation (1), . 0045 = [CO 2 3 ] + 2 . 83017 10 5 [CO 2 3 ] + (1 . 04314 10 7 ) [CO 2 3 ] [CO 2 3 ] = . 0045 1 + 2 . 83017 10 5 + 1 . 04314 10 7 = 4 . 19994 10 10 mol / L ....
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H06 Buffer Solution - patino (mp25752) H06: Buffers McCord...

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