Chemistry Question

Question

Calculate the ratio of moles of acid to moles of base to create a 750 mL buffer solution with pH = 4.2 using acetic acid, CH3OOH, and acetate CH3COO. The ka for acetic acid is 1.8 x 10^(-5)

Answer 1

This problem utilizes the Henderson Hasselbalch equation:  pH = pKa + log [salt]/[acid] and they are asking you to find the [salt]/[acid], but actually the inverse.

pH = 4.2 

pKa = -log 1.8x10^-5 = 4.74

Substituting into HH equation:  4.2 = 4.74 + log [salt]/[acid]

log [salt]/[acid] = -0.5

[salt]/[acid] = 0.316

So, [acid]/[salt] or the ratio of acid to base = 1/0.316 = 3.16:1  mole ratio of CH3COOH to CH3COO-

If you want the mole ratio instead of molar ratio, just correct for the 750 ml, i.e 0.75 liters