4. The dependence of the electrode potential of the NO3- -selective electrode on the concentration of NO3- in standard solutions is as follows:

С, mol/l 10-5 10-4 10-3 10-2 10-1
E, mV 400 350 295 240 185

A weighed portion of potassium nitrate weighing 0.1000 g was dissolved in a volumetric flask with a capacity of 100 ml. The electrode potential of the NO3- selective electrode in this solution was 250 mV. Determine the mass fraction of NO3- in this substance.

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