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 4.5? Note that the concentration and/or pH value may differ from that in the first question. STRATEGY 1. Rearrange the Henderson-Hasselbalch equation to solve for the ratio of base (acetate) to acid (acetic acid), [A–]/[HA]. 2. Use the mole fraction of acetate to calculate the concentration of acetate. 3. Calculate the concentration of acetic acid. Step 1: The ratio of base to acid is 0.55. Step 2: The mole fraction of acetate is 0.35, and the concentration of acetate is 0.053. Step 3: Calculate the concentration of acetic acid.

Explanation / Answer

pH = 4.5

pKa = 4.76

concentration of buffer = 0.15 M

Henderson-Hasselbalch equation

pH = pKa + log [salt / acid]

4.5 = 4.76 + log [acetate / acetic acid]

[acetate / acetic acid] = 0.5495

[acetate + acetic acid] = 0.15 M

0.55 acetic acid + acetic acid = 0.15

concnetration of acetic acid = 0.097 M

concentration of acetate = 0.053 M

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