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 0.20 M buffer solution at pH 4.9? 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 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: The ratio of base to acid is 1.4. That is, there are 1.4 molecules of acetate for each molecule of acetic acid. Step 2: The mole fraction of acetate is 0.58, and the concentration of acetate is 0.12. Step 3: Calculate the concentration of acetic acid. [acetic acid] =

Explanation / Answer

pH= pKa+ log([salt]/[acid])

Here pH = 4.9 and pKa = 4.76

4.9=4.76+log ([A-]/[HA])
log ([salt]/[acid])=49-4.76=0.14

So ([salt]/[acid])=10^0.14=1.4

Thus the ratio of concentration acetate /acid in the buffer solution is 1.4

here the total concentration of acetate +acid =0.20M

Let us assume if concentrations of acetic acid is x , concentration of acetate is1.4 x

x+1.4 x =2.4 x =0.20

x=0.083M acetic acid

and aetate 0.20-0.083=0.117 M

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