1 Chapter 10 Acids and Bases 10.9 Buffers. 2 When an acid or base is added to water, the pH changes drastically. A buffer solution resists a change in.

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Presentation transcript:

1 Chapter 10 Acids and Bases 10.9 Buffers

2 When an acid or base is added to water, the pH changes drastically. A buffer solution resists a change in pH when an acid or base is added. Buffers

3 Absorb H 3 O + or OH - from foods and cellular processes to maintain pH. Are important in the proper functioning of cells and blood. In blood maintain a pH close to 7.4. A change in the pH of the blood affects the uptake of oxygen and cellular processes. Buffers

4 A buffer solution Contains a combination of acid-base conjugate pairs. Contains a weak acid and a salt of the conjugate base of that acid. Typically has equal concentrations of a weak acid and its salt. May also contain a weak base and a salt with the conjugate acid. Components of a Buffer

5 The acetic acid/acetate buffer contains acetic acid (CH 3 COOH) and sodium acetate (CH 3 COONa). The salt produces sodium and acetate ions. CH 3 COONa CH 3 COO - + Na + The salt provides a higher concentration of the conjugate base CH 3 COO - than the weak acid. CH 3 COOH + H 2 O CH 3 COO - + H 3 O + Large amount Large amount Buffer Action

6 The function of the weak acid is to neutralize a base. The acetate ion in the product adds to the available acetate. CH 3 COOH + OH - CH 3 COO - + H 2 O Function of the Weak Acid

7 The function of the acetate ion CH 3 COO - (conjugate base) is to neutralize H 3 O + from acids. The weak acid product adds to the weak acid available. CH 3 COO - + H 3 O + CH 3 COOH + H 2 O Function of the Conjugate Base

8 The weak acid in a buffer neutralizes base. The conjugate base in the buffer neutralizes acid. The pH of the solution is maintained. Summary of Buffer Action

9 pH of a Buffer The [H 3 O + ] in the K a expression is used to determine the pH of a buffer. Weak acid + H 2 O H 3 O + + Conjugate base K a = [H 3 O + ][conjugate base] [weak acid] [H 3 O + ] = K a x [weak acid] [conjugate base] pH = -log [H 3 O + ]

10 Calculation of Buffer pH The weak acid H 2 PO 4 - in a blood buffer H 2 PO 4 - /HPO 4 2- has K a = 6.2 x What is the pH of the buffer if it is 0.20 M in both H 2 PO 4 - and HPO 4 2- ? [H 3 O + ] = K a x [H 2 PO 4 - ] [HPO 4 2- ] [H 3 O + ] = 6.2 x x [0.20 M] = 6.2 x [0.20 M] pH = -log [6.2 x ] = 7.21