Question

Use the Henderson-Hasselbalch equation to perform the following calculation. Calculate the mass of solid sodium acetate required to mix with 100.0 mL of 0.10 M acetic acid to prepare a pH 4 buffer. (Ka=1.8x10^-5 for acetic acid)

Answer 1

pH= pKa + log[acetate]/[acid]

We need to fill in values for both pH and pKa, then we can find the value of the log term at the end.


The pKa of acetic acid is 4.76, and the pH you need the buffer to be is 4, so we end up with this equation:

4= 4.76 + log [acetate]/[acid]

So we find that :

-0.76 = log [acetate]/[acid]

if we do the antilog of both sides, we get:

.174 = [acetate]/[acid]

We know the concentration of acetic acid in the solution, and since you are not changing the volume at all, that's going to stay the same, so :

.174 = [acetate]/.1

[acetate]= .0174 M

So that's the concentration you need the acetate to have. Molarity is moles/L, we know that the volume if 100 mL or .100 L, so we can set up this equation:

.0174 M= moles/.100L

moles= .00174 moles sodium acetate needed

Now figure out the mass of that many moles of sodium acetate by multiplying by the molar mass:

.00174 moles * 82.03 g/mol = 0.143 g sodium acetate

Thank you and good luck.