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

Question

What concentrations of acetic acid (pKa=4.76) and acetate would be required to prepare a 0.10 M buffer solution at pH 4.5 ? Note that the concentration, the pH, or both values may differ from that in the first question.

Strategy

Rearrange the Henderson–Hasselbalch equation to solve for the ratio of base (acetate) to acid (acetic acid), [A?]/[HA] .

Use the mole fraction of acetate to calculate the concentration of acetate.

Calculate the concentration of acetic acid.

Step 1: Rearrange the Henderson–Hasselbalch equation to solve for [A?][HA]

The Henderson–Hasselbalch equation is

pH=pKa+log([A?][HA])

Use the Henderson–Hasselbalch equation to solve for [A?][HA] if the solution is at pH 4.5.

Explanation / Answer

pH of acidic buffer = pka + log[H3CCOONa]/[H3CCOOH]

      pka of H3CCOOH = 4.76

total concentration of buffer = 0.1 M

[CH3COONa]= x M

[CH3COOH] = 0.1-X M

pH = 4.5

4.5 = 4.76+log(x/(0.1-x))

x = 0.0355

concentration of [CH3COONa] = 0.0355 M

concentration [CH3COOH] = 0.1-0.0355 = 0.0645 M

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