141. Solving a chemistry problem

20. A weighed sample of a drug with a mass of 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 according to 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, mВ, at m = 0.1260 g 355 360 382 523 579 597 611 620
E, mВ. at m = 0.1256 g 344 357 381 516 587 604 617 -
The position of the equivalence point is determined by the Gran´s method.