Answer to Question #168595 in General Chemistry for Robel

Question

Calculate the [OH–] in a 1.0 × 10–3 M solution of the weak base methylamine

(CH3NH2), whose Kb = 4.4 × 10–4. What percentage of the base has dissociated?

Answer 1

CH₃NH₂ + H₂O = CH₃NH₃⁺ + OH^-

K_b = ([CH₃NH₃⁺][OH^-])/[CH₃NH₂] = [OH^-]²/[CH₃NH₂] = x²/(1*10^-3 - x) = 4.4*10^-4

Solving the equation we get [OH^-] = x = 0.000478856 M and percent dissociation x*100% = 0.0479%