Hi Aishawarya,

The Henderson Hasselbalch Equation is usually written

1) pH = pKa + log[A]/[HA].

Using the log rule logA/B = -logB/A, they have rewritten this as follows in the question:

2) pH = pKa - log[HA]/[A]

However, because it gives you all the known factors, you could use either 1) or 2) to solve this question.

Identifying the information given by the question:

pKa = 3.13
[HA] = [citric acid] = 5*10^-3 M
[A] = [citrate] = 5*10^-1 M

Plugging these values into the equation 1):

pH = 3.13 + log(5*10^-1M)/(5*10^-3M)

pH = 3.13 + log(10^2)

pH = 3.13 + 2

pH = 5.13, giving D as the correct answer.

What I like to do next is check to make sure I got the HA/A or A/HA part right.

In the question stated, the concentration of A is higher than the concentration of HA. This indicates that we should have a more basic solution, and as a result, our answer makes sense.

Please let me know if this clears things up.

Thanks!

Katt