Determine the number of grams of sodium acetate dihydrate you would add to 500 ML of .250M acetic acid (pKa=4.750) to make a buffer of pH 5.00. Assume no change in volume.

Answer 1

We can use the Henderson Hasselbalch equation to address this question:

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

If you look up the pKa for acetic acid, you will find a value of 4.8. Using this value, and solving for [acetate]/[acid], we have 5.00 = 4.8 + log [acetate]/[acid]

log [acetate]/[acid] = 0.2

[acetate]/[acid] = 1.22

We know the [acid] = 0.250 M, thus...

[acetate]/0.250 = 1.22

[acetate] = 0.305 M

molar mass sodium acetate dihydrate = 118 g/mol

mass sodium acetate dihydrate required = 118 g/mol x 0.305 mol/L x 0.5 L = 18.0 g