When you add a small amount of strong base to a weak

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Unformatted text preview: mental pH will be close, but not exactly equal to that predicted by the equation. Although you could then add a little more of one or the other of the buffer components to get the pH right, it is much easier to add small amounts of strong acid or strong base, which will shift the equilibrium in the buffer a little, changing the ratio of buffer components, to get the pH exactly where you want it. 21 Friday, March 1, 13 So far we’ve mentioned two ways to make a buffer: A weak acid + a salt of that acid (containing its conjugate base) A weak base + a salt of that base (containing its conjugate acid) There is another way to make a buffer: A weak acid or base + a strong base or acid. When you add a small amount of strong base to a weak acid, the base will neutralize the acid until all the base is used up: OH–(aq) + HA(aq) → H2O(aq) + A–(aq) If the amount of strong base you add is not enough to neutralize all the weak acid, then you are left with a solution that contains both the weak acid and its conjugate base. Which is a buffer. 22 Calculation Time A buffer solution is made by dissolving 0.45 moles of a weak acid (HA) and 0.33 moles of KOH into 580 mL of solution. What is the pH of this buffer? (Ka = 7.4 × 10–6) When we add KOH (strong base), it fully dissolves and dissociates. The OH– will react with weak acid HA, reducing [HA] and increasing [A–]. HA(aq) + KOH(aq) ← K+(aq) + A–(aq) + H2O → Initial Change Final HA 0.45 mol – 0.33 mol 0.45 – 0.33 = 0.12 mol OH– 0.33 mol – 0.33 mol 0 A– 0 + 0.33 mol 0.33 mol [0.33 mol/0.58L] pH = pKa + log [Base] = –log(7.4 × 10–6) + log = 5.5701 [0.12 mol/0.58L] [Acid] 23 Buffer Capacity Think about how a buffer works. Here is a weak acid/conjugate base buffer: HA(aq) + H2O(aq) ← H3O+(aq) + A–(aq) → A–(aq) + H2O(aq) ← OH–(aq) + HA(aq) → When strong acid is added to it, it reacts with the conjugate base, reducing the amount of conjugate base and increasing the amount of weak acid. Le Chatelier tells us that this will drive the acid equilibrium further to the right, making more conjugate base and using up some of the extra weak acid that has been made (the common ion effect). In this way the buffer self corrects and maintains the pH. You can only keep doing this for a set amount of time before the buffer fails. 24 Friday, March 1, 13 The buffer capacity of a buffered solution is defined as how many hydronium ions or hydroxide ions can it absorb without significantly changing the pH of the solution; i.e., how well does it hold the pH. A high capacity buffer can withstand many additions of acid or base without losing its buffering power. We have seen that the pH of a buffer is determined by the ratio of base to acid concentration. The capacity of a buffer is determined by the magnitudes of base and acid concentration. The higher the concentrations, the higher the capacity. Recall that we said a buffer of 5 M acetic acid/3 M sodium acetate will be the same pH as a buffer of 0.1 M acetic acid/ 0.06 M sodium acetate. This is true, but the higher concentration buffer can withstand more pH challenges than the lower concentration buffer. 25 Endnotes Office hours before next lecture: Monday: Ruth 10 am, Susan 11 am Sample exam will be posted by 5 pm this afternoon. ALEKS Obj 7 due Monday 1 pm Don’t forget exam reviews: Susan: Monday 4–5 pm. WEL 1.316 Heather: Tuesday 4–5 pm. Gearing 105 DrRuth: Monday, in class No Textbook reading or feedback for Monday (exam review) Friday, March 1, 13 26...
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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 at Austin.

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