Answer is: pH of methylamine
is 11.78.
Chemical reaction: CH₃NH₂(aq)+
H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH⁻(aq).
Kb(CH₃NH₂) =
4,4·10⁻⁴.
c₀(CH₃NH₂) = n(CH₃NH₂) ÷ V(CH₃NH₂).
c₀(CH₃NH₂) = 0.070 mol ÷ 0.8 L = 0.0875 M.
c(CH₃NH₃⁺) = c(OH⁻) = x.
c(NH₂OH) = 0.0875 M - x; equilibrium concentration of methylamine.
Kb = c(CH₃NH₃⁺) · c(OH⁻) / c(CH₃NH₂).
0.00044 = x² / (0.0875 M - x).
Solve quadratic equation: x = c(OH⁻) = 0.00598 M.
pOH = -log(0.00598 M) = 2.22.
pH = 14 - 2.22 = 11.78.
