Chem 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). The given answer is 0.288 moles of base to 1 mole of acid. How do I get to this answer? Thank you so much!

Answer 1

This is a buffer problem, so you should become intimately familiar with the Henderson Hasselbalch equation.

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

pH = 4.2

pKa = -log Ka = -log 1.8x10^-5 = 4.74

Substituting into the HH equation we get ...

4.2 = 4.74 + log [salt]/[acid]

log [salt]/[acid] = 4.2 - 4.74 = -0.54

[salt]/[acid] = 0.286

NOTE: this is the ratio of salt (base) to acid, not the ratio of acid to base.

You can check this by using the ratio and solving for the pH.  Thus...

pH = 4.74 + log [base]/[acid] = 4.74 + log 0.286

pH = 4.74 + (-0.54)

pH = 4.2

So, once again, I have to disagree with the answer given by your professor.

The ratio of acid to base would be 1/0.286 = 3.5 moles base to 1 mole acid