A one molar solution of the weak base methylamine, CH3NH2 is diluted with water until the concentration...

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Chemistry

A one molar solution of the weak base methylamine,CH₃NH₂ is diluted with water until theconcentration is Â½ molar. Which of the following statements arecorrect 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 & Explanation Solved by verified expert

3.9 Ratings (816 Votes)

Answer : C) 1 & 2 only

Explanation :

CH3NH2Â Â Â +Â Â H2OÂ Â Â ------------->Â Â CH3NH3+ +Â Â OH-

Â Â 1Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0

1 - xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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

CH3NH2Â Â Â +Â Â H2OÂ Â Â ------------->Â Â CH3NH3+ +Â Â OH-

Â Â 0.5 Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0

0.5- xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 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.

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