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 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.

[acetate] = ____ M

Explanation / Answer

pKa = 4.76

pH = 5

buffer concentration = 0.15

[acetic acid] + [acetate] = 0.15M -------------------->1

Henderson-Hasselbalch equation

pH = pKa + log [aceate /acetic acid]

5 = 4.76 + log [aceate /acetic acid]

0.24 = log [aceate /acetic acid]

[aceate /acetic acid] = 1.738

[aceate ] = 1.738 x [acetic acid] ---------------------> 2

from 1 &2

[acetic acid]   + 1.738 x [acetic acid] = 1.5

[acetic acid] = 0.55 M

[aceate ]   = 0.95 M

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