Weak Acids and Bases

Question 1

Kw = Ka * Kb under what condition?

Answer 1

Only for conjugate acid-base pairs

Question 2

A- (aq) + H2O (l) ⇌ HA (aq) + OH- (aq)
What is this reaction called?

Answer 2

The general base dissociation reaction

Question 3

HA (aq) + H2O (l) ⇌ A- (aq) + H3O+ (aq)
What is this reaction called?

Answer 3

The general acid dissociation reaction

Question 4

As initial acid concentration decreases, does the percent dissociation of the acid increase or decrease?

Answer 4

Increases. Adding more H2O to make the solution more dilute shifts the reaction to the right, forming more dissociated products.

Question 5

For a general acid, what is the formula for % dissociation?

Answer 5

[H3O+]/[HA]initial * 100 or
[A-]/[HA]initial * 100

Question 6

For polyprotic acids, does most H3O+ generated come from the 1st dissociation or subsequent dissociations?

Answer 6

1st dissociation

Question 7

What is pKa?

Answer 7

-log(Ka)

Question 8

How do you find Kb from pKb?

Answer 8

Kb = 10^(-pKb)

Question 9

Does a stronger base have a higher or lower pKb?

Answer 9

Lower
Strong base –> High Kb –> low pKb

Question 10

For B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq), what is the base-dissociation expression?

Answer 10

Kb = [BH+][OH-]/[B]

Question 11

Does the base-dissociation constant actually describe the dissociation of a base?

Answer 11

No, it describes a base accepting a proton from H2O.

Question 12

What are the two main classes of weak bases in aqueous solution?

Answer 12

1. Ammonia & amines
2. Anions of weak acids

Question 13

The autoionization of water adds very small quantities of which species to an aqueous solution?

Answer 13

OH- and H3O+

Question 14

For a conjugate acid-base pair, pKa + pKb = ?

Answer 14

pKa + pKb = 14