159_lect_16_acid_bases_dissoc-738

159_lect_16_acid_bases_dissoc-738 - 3/24/2008 Lecture 16...

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3/24/2008 1 All Rights reserved Gene Hall 1 Lecture 16 Outline of Chapter 18 J 18.5 Weak bases J Examples of weak bases J 18.6 Molecular Properties and Acid Strength J Acidity of hydrated metal ions J 18.7 Acid-Base Properties of Salt Solutions J What is the pH of solutions of different salts? All Rights reserved Gene Hall 2 Some shortcut calculations that were derived from the definitions of weak acids and bases. L For weak acids: L Valid when [HA]/K a >10 3 . This means that the relative error will be 1.6%. So, when [HA]/K a > 10 3 , can use this equation. ( 29 [H O ] = [HA] K pH = -log [HA] K 3 + a a
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3/24/2008 2 All Rights reserved Gene Hall 3 Calculating pH L When the values are less than 10 3 , then solve interactively instead of the quadratic formulae: L For weak acid: x K K K a a a 2 2 [ [ ] [ ] [ [ ] ([ [ ] [ ] until eq HA]- x x is hydronium ion and rewrite: H O H O HA] = 0; Note quadratic formulae H O K HA]- H O ); solve by guessing values of H O uality is met. 3 + 3 + 3 + a 3 + 3 + = + - = All Rights reserved Gene Hall 4 Find pH 0.15M chloroacetic acid K a = 1.36E-3 L CH 2 ClCO 2 H (aq) + H 2 O (l) = H 3 O + (aq)+ CH 2 ClCO 2 - (aq) L A B CA CB L Start 0.15 - 1E-7 - L Change –x x x L Equil: 0.15-x x+1E-7 x L Now: [HA]/K a =110 which is < 1E3 and solve iteratively by guessing (logical) Let x = amount acid that dissociates = [H 3 O + ]
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3/24/2008 3 All Rights reserved Gene Hall 5 Acid pH calculations continued: L Try 0.01359, gives 0.01362. Notice not changing so we keep 0.0136 L [H 3 O + ]=0.0136 L pH = 1.87 [ ] ([ [ ] [ ] [ [ ] ([ [ ] H O K HA]- H O ) can be simplified for first guess H O K HA] then use this guess to put back in H O K HA]- H O ) until there is no change. 3 + a 3 + 3 + a 3 + a 3 + = = = 0.0142 0.01359 All Rights reserved Gene Hall 6 Bases
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4 All Rights reserved Gene Hall 7 Weak bases and K b L We can write base dissociation constants for bases just as we did for weak acids. L
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This note was uploaded on 04/03/2008 for the course CHEM 159-160 taught by Professor Zbaida during the Fall '07 term at Rutgers.

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159_lect_16_acid_bases_dissoc-738 - 3/24/2008 Lecture 16...

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