35. Solving a chemistry problem

20. A weighed portion of a medicinal product weighing m(g) containing amidopyrine (M=231.3 g/mol) and indifferent impurities was dissolved in anhydrous acetic acid, dichloroethane was added and titrated with 0.1 M (K=0.9109) HClO4 in glacial acetic acid.
Calculate the mass fraction (%) of the main substance in the preparation from the following results of potentiometric titration:
V(HClO4), ml 3.80 4.00 4.20 4.40 4.60 4.80 5.00 5.20
E, mV at m=0.1256 344 357 381 516 587 604 617
The position of the equivalence point is determined by the Gran method.