AP Chemistry : PH and Pka

Study concepts, example questions & explanations for AP Chemistry

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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?

Possible Answers:

H2A+

HA

H3O+

Equal amounts of acid and conjugate base are present.

A­-

Correct answer:

HA

Explanation:

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?

Possible Answers:

Equal amounts of acid and conjugate base are present.

A­-

HA

H3O+

H2A+

Correct answer:

A­-

Explanation:

Since pH > pKA, the deprotonated form of the acid is predominant.

Example Question #2 : 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.

Possible Answers:

H2CO3

H3O+

CO32-

HCO3-

Can not be determined.

Correct answer:

H2CO3

Explanation:

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?

Possible Answers:

H3O+

Can not be determined

SO32-

HSO3-

H2SO3

Correct answer:

HSO3-

Explanation:

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?

Possible Answers:

H2PO4-

H3PO4

PO43-

HPO42-

Can not be determined

Correct answer:

PO43-

Explanation:

Since pH > pKa1, pKa2, and pKa3 the predominant form of the acid is the fully deprotonated form.

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