L19_AcidBase6_115b08

# L19_AcidBase6_115b08 - L19: The buffer formula: [ A- ]...

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L19: The buffer formula: Henderson- Hasselbalch Equation Buffer pH: obtained from pK a and the ratio [HA] / [A - ] Now add small amounts of acid or base to buffer Buffer reactions: HA, A - : H + + A - Æ HA OH - + HA Æ H 2 O + A - B, BH + : H + + B Æ BH + OH - + BH + Æ B + H 2 O A basic buffer: NH 3 and NH 4 Cl An acidic buffer: Acetic acid and sodium acetate [] log a HA pH pK A ≈− log a acid pH pK base

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New initial values of [HA] and [NaAc] are calculated by soichiometry after a strong acid or base are added, and then ICE is used to find new equilibrium concentrations. Generalize: Divide the problem into two steps Add only a small concentration of strong acid or base. 1. Assume the first step is stoichiometric (goes to completion): H + + A - Æ HA or OH - + HA Æ H 2 O + A- 2. Let equilibrium re-establish itself [] log a HA pH pK A ≈−
Buffer reactions: HA, A-: H + + A - Æ HA OH - + HA Æ H 2 O + A - B, BH+: H + + B Æ BH + OH - + BH + Æ B + H 2 O Buffered mixtures occur in titrations of weak acid with strong base: HAc ( aq ) + NaOH ( aq ) Æ NaAc ( aq )+±H 2 O ( l ) Æ Na + ( aq ) + Ac - ( aq 2 O ( l ) If HAc is a weak acid then Ac - ( aq ) is a strong base that exists in equilibrium: Ac - ( aq 2 O ( l ) HAc ( aq ) + OH - ( aq )

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## This note was uploaded on 07/22/2008 for the course CHEM 115b taught by Professor Reisler during the Spring '08 term at USC.

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L19_AcidBase6_115b08 - L19: The buffer formula: [ A- ]...

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