What concentrations of acetic acid (pK_a = 4.76) and acetate would be required t

Question

What concentrations of acetic acid (pK_a = 4.76) and acetate would be required to prepare a 0.15 M buffer solution at pH 4.9? 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]. Use the mole fraction of acetate to calculate the concentration of acetate. Calculate the concentration of acetic acid Rearrange the Henderson Hasselbalch equation to solve for [A^-1]/[HA] The Henderson-Hasselbalch equation is pH = pK_a + log [[A^-]/[HA]] Use the Henderson-Hasselbalch equation to solve for [A^-]/[HA] if the solution is at pH 4

Explanation / Answer

pH = 4.9

pKa = 4.76

so..

pH = pKa + log(A-/HA)

solve for ratio

4.9 = 4.76 + log(ratio)

ratio = 10^(4.9-4.76) = 1.380384

concentations:

[A-]/[HA] = 1.380384

[A-] = 1.380384*[HA]

since

[A-] + [HA] = 0.15

solve

[A-] = 1.380384*[HA]

1.380384*[HA] + [HA] = 0.15

2.380384[HA] = 0.15

[HA] = 0.15 / 2.380384 = 0.0630 M

so

[A-] = 1.380384*[HA] = 1.380384*0.0630 = 0.086964192 M

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