Question

An acid HA has a pKa of 6.2, at pH 4, which form dominates, HA or A-? Write only HA or A- I am looking to learn how to do this calculation, so it would be awesome if you could show your work! :)

Answer 1

To show the form which dominates which use Henderson equation since the given system is buffer of weak acid HA and its conjugate base. A-

Equation :

pH = pka + log ([A^-]/[HA])

If the rate ([A^-]/[HA]) is equal to 1 then we say that both are in equal amount. If it is less than one then we say that acid form dominates. And when its value is greater than 1 then we say that its conjugate base form dominates.

Lets plug in the values given

4 = 6.2 + log ([A^-]/[HA])

Log ([A^-]/[HA]) = 4 – 6.2

Lets take antilog of both side.

([A^-]/[HA]) = 0.0063 , which is less than 1.

Therefore, the answer is dominating species is HA