# • A buffer contains equal amounts of a weak acid and its...

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Buffered Solutions Resist a change in pH when either H + or OH - is added Consist of solutions of weak acids or bases containing a common ion
What is a Buffered Solution? A buffered solution resists changes in pH when either H + or OH - are added It consists of either a mixture of 1. A weak base + its conjugate acid or 2. A weak acid + its conjugate base
What is a Buffered Solution? Possibility 1: a weak base + its conjugate acid + + + 4 3 NH NH H O H NH OH NH 2 3 - 4 + + + The weak base _____________ The conjugate acid ______________
What is a Buffered Solution? Possibility 2: a weak acid + its conjugate base O H O H C O H HC OH 2 2 3 2 2 3 2 + + 2 3 2 - 2 3 2 O H HC O H C H + + The weak acid _____________ The conjugate base ____________
Adding an Acid to Water vs to a Buffered Solution (aq) (aq) (aq) Cl H HCl + + Conc. pH H + H + (aq) 2 3 2 (aq) 2 3 2 (aq) O H HC O H C H + + H + HC 2 H 3 O 2 C 2 H 3 O 2 - Conc. pH (aq) (aq) (aq) Cl H HCl + + H + HC 2 H 3 O 2 C 2 H 3 O 2 -
How Does Buffering Work? HA A H + + H + not allowed to accumulate (reacts with A - ) pH does not change significantly H + Added H + ions replaced by HA
Adding a Base to Water vs to a Buffered Solution Conc. pH OH - HC 2 H 3 O 2 C 2 H 3 O 2 - Conc. pH OH - HC 2 H 3 O 2 C 2 H 3 O 2 - (aq) (aq) (aq) OH Na NaOH + + (aq) (aq) (aq) OH Na NaOH + + OH - OH - (aq) 2 3 2 2 (aq) 2 3 2 (aq) O H C O H O H HC OH + +
How Does Buffering Work? O H A HA OH 2 + + OH - not allowed to accumulate (reacts with HA) pH does not change significantly
Buffered Solutions A buffer contains equal amounts of a weak acid and its conjugate base and has a pH of 5.25. Which would be a reasonable value of buffer pH after the addition of a small amount of acid?
+ = A HA K H a Henderson-Hasselbalch Equation + = A HA log logK H log a HA A log logK H log - a + = + HA A log pK pH - a + = Only valid for buffers where the relative amounts of acid and base do not differ by more than 10:1
pH of a Buffer Solution HC 2 H 3 O 2 ( aq ) H + (aq) + C 2 H 3 O 2 - ( aq ) Initial 0.20 M ~0 0.30 M Change x +x +x Equilibrium 0.20 x x 0.30 + x Calculate the pH of a solution containing 0.20 M HC 2 H 3 O 2 (Ka = 1.8 x 10 -5 ) and 0.30 M NaC 2 H 3 O 2 Approach 1    5 - a x10 8 . 1 x - 0.20 x 0.30 x HA A H K = + = = + Assume x is small => x = 1.2x10 -5 => pH = 4.92
pH of a Buffer Solution C 2 H 3 O 2 - ( aq ) + H 2 O HC 2 H 3 O 2 ( aq ) + OH - ( aq ) Initial 0.30 M 0.20 M ~0 Change x +x +x Equilibrium 0.30 x 0.20 + x x Calculate the pH of a solution containing 0.20 M HC 2 H 3 O 2 (Ka = 1.8 x 10 -5 ) and 0.30 M NaC 2 H 3 O 2 Approach 2   10 - 5 - 14 - b x10 6 . 5 x10 8 . 1 x10 0 . 1 x - 0.30 x x 0.20 A OH HA K = = + = = Assume x is small => x = 8.3x10 => pH = 4.92
pH of a Buffer Solution Calculate the pH of a solution containing 0.20 M HC 2 H 3 O 2 (Ka = 1.8 x 10 -5 ) and 0.30 M NaC 2 H 3 O 2 Approach 3 92 . 4 0.20 0.30 log 74 . 4 HA A log pK pH - a = + = + = pH = 4.92
pH of a Buffer Solution HC 2 H 3 O 2 ( aq ) H + (aq) + C 2 H 3 O 2 - ( aq ) Initial 0.20 M ~0 0 Change x +x +x Equilibrium 0.20 x x x What would be the pH of a 0.20 M HC 2 H 3 O 2 solution if no salt were present?  5 - 2 a x10 8 . 1 x - 0.20 HA A H K = = = + x Assume x is small => x = 1.9x10 -3 => pH = 2.72
Henderson-Hasselbalch Equation
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