Buffer solution

Question

What mass of sodium acetate must be added to 2.50 L of 0.55 M acetic acid to make a buffer solution with pH = 5.75? pK (HAC)=4.75, molar mass of NaAc is 82 g/mol (Ignore volume change after solid added).

Answer 1

We can use the Henderson Hasselbalch equation for this:

pH = pKa + log [conj.base] / [acid]

pH = 5.75

pKa = 4.75

[acid] = 0.55 M

[conj.base] = ?

5.75 = 4.75 + log (x/0.55) where x = [conj.base] = [NaAc]

log (x/0.55) = 1.00

x/0.55 = 10

x = 0.055 M

Since we have 2.50 L, this means we need 0.055 mols / L x 2.50 L = 0.1375 moles of sodium acetate

0.1375 mols NaAc x 82 g / mol = 11.3 g of NaAc needed (3 sig. figs.)