39(c). Coulometric titration of the analyzed solution of hydrochloric acid with a volume of V(HCl) by electrogenerated hydroxide ion was carried out at constant current i in the presence of the phenolphthalein indicator. The titration was completed at the moment when the color of the solution changed after the time τ. A blank titration under the same conditions took τ0 seconds.
Write the equations for the reactions taking place on the generator electrode and in the solution. From the data below, calculate the molar concentration of the analyzed HCl solution.

V(HCl), ml i, mA τ τ0
10.00 5 4 min 20 s 10 s

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