What concentrations of acetic acid (pKa = 4.76) and acetate would be required to

Question

What concentrations of acetic acid (pKa = 4.76) and acetate would be required to prepare a 0.15 M buffer solution at pH 4.6? Note that the concentration and/or pH value may differ from that in the first question. STRATEGY Rearrange the Henderson-Hasselbalch equation to solve for the ratio of base (acetate) to add (acetic add), [A_]/[HA]. Use the mole fraction of acetate to calculate the concentration of acetate. Calculate the concentration of acetic add That is of base to acid is [acetate]/[acetic acid]=0.69 That is, there are 0.69 molecules of acetate for each molecule of acetic acid. Use the mole fraction of acetate to calculate the concentration of acetate.

Explanation / Answer

acetate (A-) + acetic acid (HA)= 0.15 M -------------------> 1

pH = pKa + log [acetate /acetic acid]

4.6 = 4.76 + log [A- /HA]

[A- /HA] = 0.69

[A-] = 0.69 [HA] ------------------------>2

0.69 HA + HA = 0.15

[HA] = 0.089 M

[A-] = 0.061 M

[acetic acid] = 0.089 M

[acetate] = 0.061 M

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