chapter-19 lecture-2

chapter-19 lecture-2 - How to keep a nearly constant pH? A...

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How to keep a nearly constant pH? A buffer must contain an acid component that can react with the added OH - ion, and a basic component that can react with added H 3 O + ion Cannot be simple mixture of acid and base—they would neutralize! Most often, the components of a buffer are the conjugate acid-base pair of a weak acid (or base) p( ) – for example, acetic acid (CH 3 COOH) and acetate ion (CH 3 COO - ) How a buffer works: The common-ion effect CH 3 COOH( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) If we dissolve acetic acid in a sodium acetate solution: CH 3 COO - is called the common ion : • It is common to both the acetic acid and sodium acetate solutions 19-4 • It suppresses the dissociation of CH 3 COOH
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Table 19.1 he Effect of Added Acetate Ion on the Dissociation of Acetic Acid The Effect of Added Acetate Ion on the Dissociation of Acetic Acid CH 3 COOH( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) [CH 3 COOH] initial [CH 3 COO - ] added % Dissociation* pH 10 00 3 89 0.10 0.00 0.10 0.050 1.3 0.036 2.89 4.44 0.10 0.10 0.10 0.15 0.018 0.012 4.74 4.92 * % Dissociation = [CH 3 COOH] dissoc [CH 3 COOH] initial x 100 19-5 The common ion, CH 3 COO-, suppresses the dissociation of CH 3 COOH
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How a buffer works CH 3 COOH( aq ) + H 2 O( l ) CH 3 COO - ( aq ) + H 3 O + ( aq ) = [CH 3 COO - ][H 3 O + ] + = [CH 3 COOH] Buffer with equal K a [CH 3 COOH] [H 3 O ] K a [CH 3 COO - ] constant u e equa concentrations of conjugate base and acid Buffer after addition of H 3 O + Buffer after addition of OH - OH - H 3 O + H O + CH COOH H O + + CH COO - CH COOH + OH - CH COO - + H O 19-6 2 3 3 3 3 3 2 The buffer components consume virtually all the added H 3 O + and OH - without significantly changing the component concentration ratio.
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Sample Problem Calculating the Effect of Added H 3 O + or OH - on Buffer pH ROBLEM: alculate the pH: PROBLEM: Calculate the pH: (a) of a buffer solution consisting of 0.50 M CH 3 COOH and 0.50 M CH 3 COONa (b) after adding 0.020 mol of solid NaOH to 1.0 L of the buffer solution in part (a) (c) after adding 0.020 mol of HCl
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chapter-19 lecture-2 - How to keep a nearly constant pH? A...

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