J.R. S. answered • 05/11/18

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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}= 4.74Substituting 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