CH3NH2(aq) + H2O(l) ⇄ CH3NH3+(aq) + OH−(aq) Kb = 4.4 × 10−4 Methylamine, CH3NH2, is a weak base that — Equilibrium Chemistry Question

Kb = [CH₃NH³⁺][OH^-] / [CH₃NH₂]

One point is earned for the correct expression.

pH = 11.77
[H⁺] = 10^-11.77 = 1.7 × 10^-12
[OH^-] = Kw / [H⁺] = 1.0 × 10^-14 / (1.7 × 10^-12) = 5.9 × 10^-3

OR

pOH = 14 − pH = 2.23
[OH^-] = 10^-2.23 = 5.9 × 10^-3

One point is earned for correct [OH^-].

Kb = [CH₃NH³⁺][OH^-] / [CH₃NH₂]
4.4 × 10^-4 = (5.9 × 10^-3)(5.9 × 10^-3) / (x - 5.9 × 10^-3)
x = 0.085 M

One point is earned for [CH₃NH³⁺] = [OH^-].
One point is earned for the correct initial molar concentration.
Note: An approximated molar concentration does not earn the second point.

CH₃NH₂ + H₃O⁺ → CH₃NH³⁺ + H₂O
OR
CH₃NH₂ + H⁺ → CH₃NH³⁺

One point is earned for a correct equation.

0.085 mol / 1000. mL × 50.0 mL = 0.00425 mol CH₃NH₂
0.00425 mol HCl / 36.0 mL × 1000. mL / 1.000 L = 0.12 M

One point is earned for equal moles of acid and base.
One point is earned for the correct concentration.

One point is earned for a curve starting at a pH between 11 and 12.
One point is earned for labeling the equivalence point at V = ~36.0 mL HCl and a pH of ~5.98 .
One point is earned for general shape of the curve for a weak acid/strong base titration.