BCHE 395-3 2013 pH Buffers Titrations

1 moles of aceyc acid is added to water to a nal

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Unformatted text preview: of the weak acid plus its conjugate base Buffers •  Ex. Common laboratory buffer TRIS –  Tris(hydroxymethyl)aminomethane •  Protonated Tris (R- NH3+) is a weak acid (HA) •  It is a conjugate acid because it dissociates to a weak base (the conjugate base: A- ) •  R- NH3+ ⇔ R- NH2 + H+ •  Buffers are mixtures of conjugate acid and conjugate base pairs Buffers •  So R- NH3+ (conjugate acid) becomes a reservoir to “sop up” added hydroxyl ions (OH- ) •  R- NH3+ + OH- ⇔ R- NH2 + H2O –  So OH- is “neutralized” in a buffer to water •  R- NH2 (conjugate base) becomes a reservoir to “sop up” added hydrogen ions (H+) with the same effect (R- NH2 + H+ à༎ R- NH3+) •  Buffers resist pH change because of their iniYal weak dissociaYon and reservoirs of conjugate acid and base. Important. Ex. 1 What is the iniYal pH of a 0.01 M Tris- HCl soluYon? The dissociaYon constant (Ka) of Tris- H+ is 5.01 x 10- 9 + ⇔ Tris + H+ Ka = [H+][A- Tris- H (0.01- x) x Short- cut method: x [HA] (x) (x) x2 Ka = = = 5.01 x 10- 9 (0.01- x) (0.01- x) X is very small, so: 2 x ≅ 5.01 x 10- 9 à༎à༎ = 7.07 x 10- 6 Ka ≅ 0.01 pH = – log [H+] = – log (7.07 x 10- 6) = 5.15 Ex. 2 What is the final pH of a soluYon when 0.1 moles of aceYc acid is added to water to a final volume of 1L? O Ka H3C O H3 C OH 0.1 – x Ka = + O- x H+ x [ x ][x ] = 1.74 ⋅10 −5 M [0.1 - x] x 2 = 1.74 ⋅10 −6 − 1.74 ⋅10 −5 x x 2 + 1.74 ⋅10 −5 x − 1.74 ⋅10 −6 = 0 x = 0.001310, pH = 2.883 • ...
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