At which pH would a buffer with a pKa of 754 work well 678 1

At which pH would a buffer with a pKa of 7.54 work well?
   6.78
   12.47
   9.53
   2.45

Solution

Answer: (a) 6.78

Following Henderson–Hasselbalch equation,

pH = pKa + log [B/A], where B = Conjugate base anion, A = Weak acid cation

The buffer is most effective when pH = pKa, that is at [B/A] = 1

pH = 6.78 is closer to the pKa value of 7.54.

So, at pH 6.78, the buffer should work well.