Ch 7_Part 2_112011 - 1 11/20/2011 Water itself is a weak...

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Unformatted text preview: 1 11/20/2011 Water itself is a weak acid and weak base: Two water molecules react to form H 3 O + and OH- (but only slightly) H 2 O H 2 O H 3 O + OH- Autoionization of Water H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH- (aq) K = [H 3 O + ][OH- ] [H 2 O] 2 The ion-product for water, K w : K = K w = [H 3 O + ][OH- ] = 1.0 x 10-14 (at 25 o C) For pure water the concentration of hydroxyl and hydronium ions must be equal: [H 3 O + ] = [OH- ] = 1.0 x 10-14 = 1.0 x 10-7 M (at 25 o C) Set = 1.0 since nearly pure liquid water even when salts, acids, or bases present. = [H + ][OH- ] = K w 2 11/20/2011 The Meaning of K b , the Base Dissociation Constant For the generalized reaction between a base, B(aq), and water: B (aq) + H 2 O (l) B H + (aq) + OH- (aq) Equilibrium constant: The stronger the base, the higher the [OH- ] at equilibrium and the larger the K b . K = K b = [BH + ][OH- ] [B] The Relation Between K a and K b for a Conjugate Acid-Base Pair Acid HA + H 2 O H 3 O + + A- Base A- + H 2 O HA + OH- 2 H 2 O H 3 O + + OH- x = [H 3 O + ] [OH- ] [H 3 O + ] [A- ] [HA] [HA] [OH- ] [A- ] K a x K b = K w For HNO 2 : K a = 4.5 x 10-4 and for NO 2- : K b = 2.2 x 10-11 K a x K b = (4.5 x 10-4 )(2.2 x 10-11 ) = 9.9 x 10-15 or ~ 10 x 10-15 = 1 x 10-14 = K w 3 11/20/2011 pH of a Strong Acid Calculate pH of 1.0 M HClO 4 major species : H + , ClO 4- and H 2 O perchloric acid is a strong acid: FULLY dissociated Therefore, 1.0 M HClO 4 1.0 M H + pH = - log [H + ] = - log [1.0] = 0.00 Calculate pH of 0.01 M HClO 4 pH = - log [H + ] = - log [0.01] = 2.0 Calculate pH of 0.02 M HClO 4 pH = - log [H + ] = - log [0.02] = 1.7 Remember: K w = [H+][OH-] so that [OH-] = K w / [H+] 1) pOH = -log [OH-] = - log (K w /[H + ]) = - log K w (- log [H + ]) = pK w pH = - log (1.0 x 10-14 ) pH pOH = 14.00 pH 2) It is convenient to express equilibrium constants for acid dissociation in the form pK a = - log K a Two New Definitions 4 11/20/2011 pH of a Strong Base Calculate pH of 1.0 M NaOH major species : Na + , OH- and H 2 O sodium hydroxide is a strong base: FULLY dissociated Therefore, 1.0 M NaOH 1.0 M [OH- ] pH = - log [H + ]; pOH= - log [OH- ]; pK w = pH + pOH = 14 pH = 14 pOH pH = 14 (-log 1.0) = 14.0 Calculate pH of 0.01 M NaOH pH= 14 (-log 0.01) = 14 2 = 12.0 Calculate pH of 0.02 M NaOH pH= 14 (-log 0.02) = 14 - 1.70 = 12.3 Calculating [H 3 O + ], pH, [OH- ], and pOH Problem: A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M . Calculate the [H 3 O + ], pH, [OH- ], and pOH of the two solutions at 25 o C. Solution: (a) [H + ] = 3.0 M pH = -log[H + ] = -log(3.0) =-0.48 [OH- ] = = = 3.3 x 10-15 M K w [H + ] 1 x 10-14 3.0 pOH = - log(3.3 x 10-15 ) = 14.52 (b) [H + ] = 0.0024 M pH = -log[H + ] = -log(0.0024) = 2.62 pOH = -log(4.2 x 10-12 ) = 11.38 [OH- ] = = = 4.2 x 10-12 M K w [H + ] 1 x 10-14 0.0024 5 11/20/2011 pH= -log[H + ] pOH= - log [OH- ] pK w = 14 = pH + pOH K w = [H...
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This note was uploaded on 01/18/2012 for the course CHEM 142B taught by Professor John during the Fall '11 term at University of Washington.

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Ch 7_Part 2_112011 - 1 11/20/2011 Water itself is a weak...

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