A diamine is a compound that contains two amino groups and can accept two protons since each nitrogen in the compound can accept a proton. Propylenediamine (1-2-Diaminopropane), H₂NC₃H₆NH₂, is an example of a diamine:

H₂NC₃H₆NH₂(aq) + H₂O(l) ⇌ H₃NC₃H₆NH₂⁺(aq) + OH⁻(aq)          K_b1 = 6.61×10⁻⁵

H₃NC₃H₆NH₂⁺(aq) + H₂O(l) ⇌ H₃NC₃H₆NH₃²⁺(aq) + OH⁻(aq)       K_b2 = 4.07×10⁻⁸

Determine the pH of the solution after the addition of the following volumes of 0.300 M HCl to 30.0 mL of 0.200 M propylenediamine.

(i) 0 mL

(ii) 10.0 mL

(iii) 20.0 mL

(iv) 30.0 mL

(v) 40.0 mL

(vi) 50.0 mL