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 0.10 M buffer solution at pH 5.0? 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 Step 2: Use the mole fraction of acetate to calculate the concentration of acetate.

Explanation / Answer

acetate = A-

acetic acid = HA

[HA] +[ A- ]= 0.10 ------------------>1

pH = pKa + log [A-/HA]

5.0 = 4.76 + log [A- / HA]

1.74= [A- / HA]

[A-] = 1.74 [HA] ---------------->2

from 1 and 2

[HA] + 1.74 [HA] = 0.10

2.74 [HA] = 0.10

[HA] = 0.0365 M

[A-] = 0.0635 M

acetic acid concentration = 0.0365 M

acetate concentration = 0.0635 M

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