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Example Questions
Example Question #1 : Ph And Pka
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?
H3O+
A-
HA
H2A+
Equal amounts of acid and conjugate base are present.
HA
Since pH < pKa the undissociated acid is the predominant form.
Example Question #2 : 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
Equal amounts of acid and conjugate base are present.
H2A+
A-
H3O+
A-
Since pH > pKA, the deprotonated form of the acid is predominant.
Example Question #3 : Ph And Pka
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
H3O+
HCO3-
Can not be determined.
CO32-
H2CO3
Since pH < pKa1, the undissociated form of carbonic acid (H2CO3) is predominant.
Example Question #4 : Ph And Pka
Sulphurous acid (H2SO3) has a pKa1 = 1.92 and a pKa2 = 7.18, if the pH = 5.3, what is the predominant species present?
H3O+
HSO3-
H2SO3
SO32-
Can not be determined
HSO3-
Since pH > pKa1 but pH < pKa2 the predominant form is one with a single hydrogen dissociated.
Example Question #5 : Ph And Pka
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?
Can not be determined
H2PO4-
H3PO4
PO43-
HPO42-
PO43-
Since pH > pKa1, pKa2, and pKa3 the predominant form of the acid is the fully deprotonated form.
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