If a buffer solution is 0.210 M in a weak acid ( 𝐾a=3.1×10−5) and 0.550 M in its conjugate base, what is the pH?

Question

If a buffer solution is  0.210 M   in a weak acid ( 𝐾a=3.1×10−5)   and  0.550 M   in its conjugate base, what is the  pH?

J.R. S. · Accepted Answer

Use the Henderson Hasselbalch equation for an acidic buffer:

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

pKa = -log Ka = -log 3.1x10^-5 = 4.51

pH = 4.51 + log (0.550 / 0.210) = 4.51 + log 2.619

pH = 4.51 + 0.418

pH = 4.93

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