CH118 ka from ph - unknown acid 453 is 1.58 x 10-4 ± 0.06...

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Determination of Ka from pH I. Validity of the Method The general formula for Ka of a weak acid is Ka= [H 3 O + ][A - ] / [HA]. The Henderson- Hasselbach equation can be used to find pKa (and therefore Ka) from pH. The Henderson- Hasselbach equation is pH = pKa + log [A - ] / [HA]. At the half equivalence point, [HA] = [A - ] = [H + ] = ½ [HA] o ,which makes pKa = pH. pKa is the negative log of Ka. In this experiment, the pH of acetic acid at the half-equivalence point was 4.52 so pKa = 4.62. To get Ka calculate 10 -4.62 , which is 2.40 x 10 -5 . The Ka of acetic acid is 2.40 x 10 -5 ± 0.09. The percent error is 75%. II. The Ka of the Acid The pH at the half equivalence point of unknown #453 was 3.80. The pKa is 3.80. The Ka of
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Unformatted text preview: unknown acid # 453 is 1.58 x 10-4 ± 0.06. III. Kb of a Weak Base The Kb of a weak base is [HA][OH-] / [A-]. To experimentally determine a Kb, I would titrate a strong acid such as HCl into a beaker with the weak base and an indicator that would change color at a low pH as opposed to a high one like phenolphthalein. The Henderson-Hasselbach equation would be pH = pKb + log [HA] / [A-] IV. Summary of the Data Acid pH at half equilibrium pH at equilibrium pKa Ka Acetic Acid 4.62 6.91 4.62 2.4 x 10-5 Unknown #453 3.8 5.22 3.8 1.58 x 10-4 V. Titration Curves...
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This note was uploaded on 06/15/2011 for the course CH 118 taught by Professor March during the Fall '09 term at University of Alabama at Birmingham.

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CH118 ka from ph - unknown acid 453 is 1.58 x 10-4 ± 0.06...

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