37. Solving a chemistry problem

4. The analyzed solution of methylamine CH3NH2 with a volume of 20.00 ml was diluted in a volumetric flask to 100.0 ml, then 10.00 ml of the resulting solution was titrated potentiometrically with 0.1000 M HCl.
Construct potentiometric titration curves in the coordinates pH - V and ΔpH / ΔV - V and determine the concentration (mol / l) of the initial solution of methylamine according to the following data:

V HCl, ml 12.00 15.00 17.00 17.50 17.80 18.00 18.20 18.50 19.00
pH 10.36 9.96 9.43 9.17 8.28 5.99 3.28 2.89 2.58