Titration Notes 2

Titration Notes 2 - Weak Acid A 33.5 mL sample of a 0.495 M...

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Weak Acid A 33.5 mL sample of a 0.495 M aqueous hypochlorous acid solution is titrated with a 0.370 M aqueous potassium hydroxide solution. What is the pH after 26.9 mL of base have been added? pH = 7.63 Feedback: Please note that while the results of the intermediate calculations below are displayed rounded to 3 significant digits, the actual calculations are done internally without rounding. When the strong base, KOH , is added, the weak acid reacts to neutralize the OH - .The balanced equation is: HClO + KOH KClO + H 2 O The initial number of moles of the weak acid HClO is given by 0.495 M 3.35E-2 L = 1.66E-2 mol Similarly, the number of moles of KOH added is given by 0.370 M 2.69E-2 L = 9.95E-3 mol Note that the amount of base added is not enough to neutralize all of the acid, so the final solution will contain both the remaining weak acid and its conjugate base. This is a buffer solution. In order to calculate the pH, calculate the moles of ClO - formed and the moles of HClO remaining. The number of moles of HClO used is 9.95E-3 mol KOH 1 mol HClO = 9.95E-3 mol HClO used 1 mol KOH The amount of HClO that remains is 1.66E-2 mol - 9.95E-3 mol = 6.63E-3 mol HClO remaining . The number of moles of ClO - formed is 9.95E-3 mol KOH 1 mol KClO 1 mol ClO - = 9.95E-3 mol ClO - formed 1 mol KOH 1 mol KClO The final solution thus contains 6.63E-3 mol HClO and 9.95E-3 mol ClO - . The pH of this buffer solution can conveniently be calculated using the Henderson-Hasselbalch equation: pH = pKa + log [ ClO - ] [ HClO ] The tabulated value of K a for HClO is 3.5E-8 . pKa = - log( 3.5E-8 ) = 7.46 Since the volumes cancel out in the ratio of molarities, substitute moles for M. Then: pH = 7.46 + log 9.95E-3 = 7.63 6.63E-3
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When a 15.6 mL sample of a 0.424 M aqueous acetic acid solution is titrated with a 0.456 M aqueous potassium hydroxide solution, what is the pH after 21.8 mL of potassium hydroxide have been added? pH =
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This note was uploaded on 09/22/2010 for the course CHEM 3 taught by Professor Pedersen during the Fall '08 term at Berkeley.

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Titration Notes 2 - Weak Acid A 33.5 mL sample of a 0.495 M...

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