16.2.4 Weak Bases

Question 1

Weak Bases

Answer 1

• The equilibrium constant for the reaction of a weak base with water is called the base-dissociation constant (K b ).
• K b values can be used to calculate equilibrium concentrations of hydroxide ion in basic solutions.
• K b for a base can be calculated from K a for its conjugate acid.

Question 2

note

Answer 2

• Weak bases only partially dissociate in aqueous solution. The equilibrium constant for the reaction of a base with water to form hydroxide ion(OH–) and the conjugate acid of the base (BH+) is the base-dissociation constant (K b ).
• K b values can be found in a K b table.
• What are the [OH–] and the pH of a 0.10 M NH 3 solution?
• First, the [OH–] is calculated using the K b .
• The negative log of the [OH–] equals the pOH.
• Subtracting the pOH from 14 yields the pH.
• The relationship between the acid-dissociation constant and the base-dissociation constant allows K b to be calculated when only K a is given.
• K a · K b = K w (K w = 1 x 10 –14 at 25°C).

Question 3

Why are extensive tables of Kb not published?

Answer 3

Kb can always be calculated from the Ka of the conjugate acid.

Question 4

What is the main difference between strong bases and weak bases?

Answer 4

• Weak bases have an equilibrium constant less than 1.0.
• Strong bases are almost completely ionized.
• The equilibrium point of a strong base B, B <==> BH+ + OH-, is far to the right

Question 5

Suppose a weak base, B, has Kb = 1.0 × 10^−8. What is the pOH and pH of a 1.5 × 10^−4 M solution of this base?

Answer 5

pOH = 5.9, pH = 8.1

Question 6

Methylamine has a Kb = 4.4 × 10^−4.
Ammonia has a Kb = 1.8 × 10^−5.
Both are weak bases. Suppose a 0.1M aqueous solution of each is prepared. Which base is relatively stronger and what will be the pH of the more basic solution?

Answer 6

Methylamine, 11.83

Question 7

Lactic acid is what makes sour milk taste sour. It is also a by-product of physical exertion and can cause sore muscles. It is a weak monoprotic acid with the formula CH3CHOHCO2H. In water, it partially dissociates, Ka = 1.4 × 10^−4, to form hydronium ions and the conjugate base CH3CHOHCO2−. Determine Kb for this base.

Answer 7

7.1 × 10^−11

Question 8

Which of the following describes weak bases?

Answer 8

A base that only partially dissociates in water

Question 9

Which of the following is TRUE regarding the carbonate ion?

Answer 9

• The carbonate ion acts as a proton acceptor
• The carbonate ion acts as a weak base
• The carbonate ion increases hydroxide ion concentration

Question 10

What is the pH of a 1.0 M solution of methylamine (CH3NH2 )? Kb = 4.4 × 10^−4

Answer 10

12.32

Question 11

Pyridine, a component of coal tar, is a base. A 1.0 × 10^−2 M solution of this base has a pH of 8.6 at 25°C. What is the equilibrium constant for this base? Is it a strong or weak base?

Answer 11

1.6 × 10^−9, weak

Question 12

What is the pH of a 10.0 M solution of ammonia (NH3 )? Kb = 1.8 × 10^−5.

Answer 12

12.11