Chapter 17 - Chapter 17 Additional Aqueous Equilibria...

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Chapter 17 Additional Aqueous Equilibria Buffers Acid-Base Titrations Solubility Equilibria Buffers A system that resists changes in pH when limited amounts of acid or base are added to it. Resists-very small changes in pH Limited by “buffer capacity” What’s needed to have a buffer? You need: a weak acid –to react with added base a weak base –to react with added acid The acid and the base can’t react with each other. Common Buffer Acetic acid/acetate is a common buffer solution CH 3 COOH(aq) + CH 3 COO - (aq) CH 3 COOH(aq) + CH 3 COO - (aq) acid base ACID BASE Buffers usually consist of approx. equal quantities of a weak acid and its conjugate base, or a weak base and its conjugate acid. Look for a weak acid/salt of a weak acid pair Or a weak base/acid of the weak base pair Buffers Present in biological systems pH of blood is regulated by buffers to 7.40±0.05 Complex mixture, but CO 2 /H 2 CO 3 system is most important Calculating the pH of a Buffer Solution Given the K a and the concentrations of the acid/base pair we can calculate the pH Several ways to do this, but the easiest is to use the Henderson-Hasselbalch equation
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Henderson-Hasselbalch Equation Derived from the K a expression for a weak acid + = + = [HA] ] [A log H acid] [conjugate base] [conjugate log H - a a pK p pK p When does pH = p K a ? log(1)=0, so when [A - ] = [HA ] t he p H = p K a How do you find this point experimentally? + = [HA] ] [A log H - a pK p Example Determine the pH of an aqueous solution that is 0.15 M in HOAc and 0.20 M NaOAc. + = [HA] ] [A log H - a pK p K a = 1.8 x 10 -5 for acetic acid 88 . 4 13 . 0 75 . 4 H (0.15) (0.20) log ) 10 * 8 . 1 log( H 5 = + = + = p p Buffer Ranges The pH range of a buffer is limited to about one pH unit above or below the pK a of the conjugate acid To make a buffer of a specific pH, you must look at the pK a of the conjugate acid Select an acid/conjugate base pair on the that has a pK a near the intended pH Using K a to Select an Acid/Base Pair for Use as a Buffer Use the Appendix to select weak acids or bases that could be used to prepare buffer solutions that have a pH of 3.00 10.70 HF/F - K a = 7.2x10 -4 Pyridine/Hpy K b = 5.0 x 10 Buffer Action A buffer contains HA and A - , when H 3 O + is added the H 3 O + reacts with the A - producing the weak acid. H 3 O + (aq) + A - (aq ) HA(aq) + H 2 O( l ) When base is added, the base reacts with the acid to produce the conjugate base, OH - (aq) + HA(aq) A - (aq) + H 2 O( l )
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Buffer Action When we are working with a weak base/conjugate acid pair… B(aq) + H 3 O + (aq) HB(aq) + H 2 O( l ) HB(aq) + OH - (aq ) B(aq) + H 2 O( l ) Buffer Capacity The buffer will not react with an unlimited amount of acid/base. You can use up HA or A - . Defn: the quantity of acid or base the buffer can accommodate without a significant pH change (more than 1 pH unit).
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This note was uploaded on 04/20/2008 for the course CHEM 115 taught by Professor March during the Spring '08 term at University of Alabama at Birmingham.

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Chapter 17 - Chapter 17 Additional Aqueous Equilibria...

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