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 4.9? 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. That is, there are 1.4 molecules the ratio of base to acid is 1.4. of acetate for each molecule of acetic acid. The mole fraction of acetate is 0.58, and the concentration of acetate is 0.058. Calculate the concentration of acetic acid.

Explanation / Answer

pH = 4.9

pKa = 4.76

total buffer concentration = 0.10 M

pH = pKa + log [acetate / acetic acid]

4.9 = 4.76 + log [acetate / acetic acid]

[acetate / acetic acid] = 1.38

[acetate + acetic acid] = 0.10 M

1.38 acetic acid + acetic acid = 0.10

acetic acid = 0.0420 M

concentration of acetic acid = 0.042 M

concentration of acetate = 0.058 M

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