Question

Calculate OH- and pH in a solution in which CH3NH2 is 0.300 M and CH3NH3+ is: (kb ch3nh2: 4.2 x 10^-4

A). 0.0500 M

B). 0.200 M

C). 0.300 M

D).0.500 M

Please show steps.

Answer 1

CH₃NH₂ dissociate as

CH₃NH₂ + H₂O CH₃NH₃⁺ + OH^-

K_b = [CH₃NH₃⁺] [OH^-] / [CH₃NH₂]

aniline is monobesic base therefore

[CH₃NH₃⁺] = [OH^-] = x

K_b = [x][x] /  [CH₃NH₂]

K_b =[x]² /  [CH₃NH₂]

[x]² = K_b  [CH₃NH₂]

[x]² = 4.2 10^-4 0.300 = 1.26 10^-4

[x] = 0.011225 M

[CH₃NH₃⁺] = [OH^-] = x = 0.011225 M

pOH = -log[OH^-] = -log(0.011225) = 1.95

pH = 14 - 1.95 = 12.05