Question

A one molar solution of the weak base methylamine, CH₃NH₂ is diluted with water until the concentration is ½ molar. Which of the following statements are correct for this process?

1. [OH^-] / [CH₃NH₃⁺] remains constant.

2. pH decreases.

3. The % ionization decreases.

• A.

  1 only

• B.

  3 only

• C.

  1 & 2 only

• D.

  2 only

• E.

  2 & 3 only

Answer 1

Answer : C) 1 & 2 only

Explanation :

CH3NH2    +   H2O    ------------->   CH3NH3+ +   OH-

   1                                                        0                  0

1 - x                                                      x                 x

Kb = x^2 / 1 - x

5 x 10^-4 = x^2 / 1 - x

x = 0.0223

[OH-] = [CH3NH3+] = 0.0223 M

[OH-] / [CH3NH3+] = 1

% ionization = 2.23 %

pH = 12.35

if concentration is = 1/2 M

CH3NH2    +   H2O    ------------->   CH3NH3+ +   OH-

   0.5                                                        0                  0

0.5- x                                                      x                 x

Kb = x^2 / 0.5 - x

5 x 10^-4 = x^2 / 0.5 - x

x = 0.0158

[OH-] = [CH3NH3+] = 0.0158 M

[OH-] / [CH3NH3+] = 1

% ionization = 3.16

pH = 12.20

here

[OH-] / [CH3NH3+] is constant

pH decreases.