Reaction of methyl amine with HCl (H+) is:
CH3NH2 + H⁺ ↔ CH3NH3⁺
At equivalence point:
moles of CH3NH2 = moles of HCl
moles of CH3NH2 = 0.350 * 0.125 = 0.0438 moles = moles of HCl
Volume of HCl = 0.0438 / 0.250 = 0.1752 L
Therefore, total volume at eq point = 0.350 + 0.1752 = 0.5252 L
Now, at equivalence point there is 0.0438 moles of CH3NH3+. The new equilibrium is :-
CH3NH3+ ↔ CH3NH2 + H+
where the concentration of CH3NH3+ = 0.0438/0.5252 = 0.083 M
Ka = [CH3NH2][H+]/[CH3NH3+]
Kw/Kb = x²/0.083-x
10⁻¹⁴/4.4*10⁻⁴ = x²/0.083-x
therefore x = [H+] = 1.37*10⁻⁶M
pH = -log[H+] = -log[1.37*10⁻⁶] = 5.86
