AP Chemistry › PH and Pka
If you have a solution that consists of a monoprotic acid (HA), with a pKa of 4.1 and at a pH of 5.8, what is the predominant species present?
HA
A-
H2A+
H3O+
Equal amounts of acid and conjugate base are present.
Since pH > pKA, the deprotonated form of the acid is predominant.
If you have a solution that consist of a weak monoprotic acid (HA), with a pKA of 5.3 and a pH of 3.2, what is the predominant species present?
HA
A-
H2A+
H3O+
Equal amounts of acid and conjugate base are present.
Since pH < pKa the undissociated acid is the predominant form.
Sulphurous acid (H2SO3) has a pKa1 = 1.92 and a pKa2 = 7.18, if the pH = 5.3, what is the predominant species present?
H2SO3
HSO3-
SO32-
H3O+
Can not be determined
Since pH > pKa1 but pH < pKa2 the predominant form is one with a single hydrogen dissociated.
Carbonic acid as a pKa1 = 6.35 and a pKa2=10.33. If the pH is 3.52, what is the predominant species present.
H2CO3
HCO3-
CO32-
H3O+
Can not be determined.
Since pH < pKa1, the undissociated form of carbonic acid (H2CO3) is predominant.
Phosphoric acid has a pKa1 = 2.15, a pKa2 = 7.20, and a pKa3 = 12.35. If the pH = 13.0, what is the predominant species present?
H3PO4
H2PO4-
HPO42-
PO43-
Can not be determined
Since pH > pKa1, pKa2, and pKa3 the predominant form of the acid is the fully deprotonated form.