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 4.6? Note that the concentration and/or pH value may differ from that in the first question. STRATEGY Rearrange the Henderson-Hasselbalch equation to solve for the ratio of base (acetate) to add (acetic add), [A_]/[HA]. Use the mole fraction of acetate to calculate the concentration of acetate. Calculate the concentration of acetic add That is of base to acid is [acetate]/[acetic acid]=0.69 That is, there are 0.69 molecules of acetate for each molecule of acetic acid. Use the mole fraction of acetate to calculate the concentration of acetate.

Explanation / Answer

Using Henderson hasselbach equation,

pH=pka+log [A-]/[HA]

5.0=4.76+log[acetate]/[acetic acid]

0.24= log[acetate]/[acetic acid]

10^0.24=[acetate]/[acetic acid]

1.7=[acetate]/[acetic acid]

But [acetate]+[acetic acid]=0.15M

1/1.7=[acetic acid]/[acetate]

0.6=[acetic acid]/[acetate]

0.6+1=[acetic acid]/[acetate]+1

1.6=[acetate]+[acetic acid]/[acetate]

Taking reciprocal,

0.63=[acetate]/[acetate]+[acetic acid]=mole fraction of acetate

Moles of acetate=0.63*0.15M=0.0945M

Conc of acetic acid=0.15M-0.0945M=0.055M

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