16.3.1 Examining Polyprotic Acids

Question 1

It is always easier to remove the first proton from a polyprotic acid than the second or third, why is this?

Answer 1

Because once the first proton is removed the acid has a net negative charge that tends to attract protons.

Question 2

Examine the following two equilibria.

H2CO3(aq) + H2O(l) H3O^+(aq) + HCO3^-(aq)

HCO3^-(aq) + H2O(l) H3O^+(aq) + CO3^2-(aq)

Which of the species (other than water) is amphoteric?

Answer 2

HCO3^-

Question 3

Carbon dioxide (CO2) is gas that reacts chemically when it dissolves in water producing carbonic acid. Carbonic acid (a diprotic acid) subsequently dissociates according to the following equation:

H2CO3(aq) + H2O(l) H3O^+(aq) + HCO3^-(aq)

HCO3^-(aq) + H2O(l) H3O^+(aq) + CO3^2-(aq)

The acid dissociation constant for the first dissociation is Ka1= 4.3 x 10^-7. What is the pH of a 0.041 M solution of carbonic acid due only to the first dissociation?

Answer 3

3.9

Question 4

Carbonic acid (a diprotic acid) dissociates in water according to the following equations:

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3^-(aq)

HCO3^-(aq) + H2O(l) H3O+(aq) + CO3^2-(aq)

The acid dissociation constants are:
Ka1 = 4.3 x 10^-7 and Ka2 = 5.6 x 10^-11.
What is [H3O+] due to the second dissociation in a 0.041 M carbonic acid solution?

Answer 4

5.6 x 10^-11

Question 5

Tartaric acid (a diprotic acid) dissociates in water according to the following equation: H2C4H4O6(aq) + H2O(l) H3O^+(aq) + HC4H4O6^-(aq) H3O^+(aq) + C4H4O6^2-(aq)

The acid dissociation constants are: Ka1= 1.0 x 10^-3 and Ka2 = 4.6 x 10^-5. What is the pH of a 1.8 M solution due to the first dissociation alone?

Answer 5

1.4

Question 6

Sulfuric acid (H2SO4 ) is a diprotic acid that has only one acid dissociation constant. Why is this?

Answer 6

Because the dissociation of the first proton is complete in this strong acid.

Question 7

Tartaric acid (a diprotic acid) dissociates in water according to the following equation: H2C4H4O6(aq) + H2O(l) H3O^+(aq) + HC4H4O6^-(aq) H3O^+(aq) + C4H4O6^2-(aq)

The acid dissociation constant are: Ka1= 1.0 x 10^-3 and Ka2= 4.6 x 10^-5. Suppose you make up a 1.8 M solution of this acid, what is the pH of the solution?

Answer 7

1.4

Question 8

Tartaric acid is used as an ingredient in an old recipe for a gold-chloride toning solution for silver chloride photographic prints. Its purpose is to make the solution more acidic. The recipe calls for a 1.3 x 10^-5 M solution (1 mg in 500 mL). The dissociation reaction for this diprotic acid is: H2C4H4O6(aq) + H2O(l) H3O^+(aq) + HC4H4O6^-(aq) H3O^+(aq) + C4H4O6^2-(aq)

The acid dissociation constants are: Ka1 = 1.0 x 10^-3 and Ka2 = 4.6 x 10^-5. What is the pH of the solution?

Answer 8

4.6

Question 9

Examine the following two equilibria

H2CO3(aq) + H2O(l) H3O^+(aq) + HCO3^-(aq)

HCO3^-(aq) + H2O(l) H3O^+(aq) + CO3^2-(aq)

Which species is the strongest base?

Answer 9

CO3^2-

Question 10

Carbonic acid (a diprotic acid) dissociates in water according to the following equations:

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3^-(aq)

HCO3^-(aq) + H2O(l) H3O+(aq) + CO3^2-(aq)

The acid dissociation constants are:
Ka1 = 4.3 x 10^-7 and Ka2 = 5.6 x 10^-11. In a 0.05 M solution of carbonic acid, what is [CO3^2-]?

Answer 10

5.6 x 10^-11 M