12 friday march 1 13 buffer calculations the best way

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Unformatted text preview: hat it is ≤5% of the number it was being added to/subtracted from. But if it is not, you’ll have to go back and redo your calculation without approximation. Unless you have a very dilute solution you can ignore water autoionization. If the final pH is between 6 and 8 (close to neutral), then you need to use the formula we provided (for dilute strong acids). 12 Friday, March 1, 13 Buffer calculations The best way to calculate the concentrations of components and/or pH of a buffer, is by treating it as an equilibrium solution and using an ICE table. The HH formula is an approximation only. It’s most commonly used to quickly determine how best to make a buffer, or how much added base/ acid will affect the buffer pH. The HH formula is based on the assumption of small change (x), same as the shortcut for our ICE tables. So if a large amount of acid/base is added to a buffer, or if the equilibrium constant of one of the reactions in the buffer is too high (meaning it isn’t a very weak acid or base), or the amount of one of the components isn’t very large, then the HH formula will mis-predict the pH of the resulting mixture. 13 Calculation Time What is the equilibrium pH of a solution which is initially mixed at 0.200 M in formic acid and 0.00500 M in formate ion? The Ka for formic acid is 1.8 × 10–4. Formic-H(aq) + H2O(l) ← H3O+(aq) + Formate–(aq) → [H3O+][Formate–] Ka = = 1.8 × 10–4 [Formic H] Formate–(aq) + H2O(l) ← Formic-H(aq) + OH–(aq) → Kb = [OH–][Formic H] 1.00 × 10–14 Kw = 5.556 × 10–11 = = [Formate–] 1.8 × 10–4 Ka The larger equilibrium constant is for the weak acid equilibrium of formic acid. Assume [H3O+] has negligible contribution from water autoionization. Initial composition Change in composition Equilibrium composition formic acid 0.200 –x formate ion 0.00500 +x 0.200 – x ≈ 0.200 0.00500 + x ≈ 0.00500 H3O+ 0 +x x (1.8 × 10–4)(0.200) [H O+][Formate–] 0.00500x Ka = 3 = = 1.8 × 10–4 x = = 7.2 × 10...
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This note was uploaded on 04/09/2013 for the course CHEM 302 taught by Professor Mccord during the Spring '10 term at University of Texas.

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