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BCHE 395-3 2013 pH Buffers Titrations

# If you understand the landmarks this concept becomes

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Unformatted text preview: We assume that the only source of H+ is the weak acid •  To ﬁnd the [H+], a quadraYc equaYon must be solved Ka is a constant. Find it in your text. Ex. 2 Cont. 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 O- OH 0.1 – x 0.1 + H+ x x Ka = x x [ x ][x ] = 1.74 ⋅10 −5 M [0.1] 2 x = 1.74 ⋅10 −6 x = 0.00132, pH = 2.880 •  The equaYon can be simpliﬁed if the amount of dissociated species is much less than the amount of undissociated acid •  ApproximaYon works for suﬃciently weak acids and bases •  Check that x < [total acid] Ex. 3 What is the ﬁnal pH when 30 mL of 0.01 M NaOH is added to 50 mL of 0.01 M Tris- HCl soluYon? •  Step 1: Determine the chemical reacYon •  Tris- H+ + OH- ⇔ Tris + H2O –  Which is the conjugate acid? Which is the conjugate base? •  Step 2: Determine the amount (in moles) of conjugate acid and conjugate base remaining aSer the reacYon IniYal Conj acid •  IniGal: •  50ml Tris- H+ X 0.01mole/1000ml = 0.5 X 10- 3 moles Tris- H+ •  Added: •  30ml NaOH X 0.01moles/1000ml = 0.3 X 10- 3 moles NaOH •  So 0.3 X 10- 3 moles of Tris form Conj base ﬁnally formed a}er OH- addiYon Ex. 3 cont. What is the ﬁnal pH when 30 mL of 0.01 M NaOH is added to 50 mL of 0.01 M Tris- HCl soluYon? •  •  •  •  •  •  •  •  Final: 50 ml + 30 ml = 80 ml total Conj acid a}er addiYon of OH 80mls with 0.2 X 10- 3 moles Tris-...
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