In standard solutions of potassium salt with a concentration of cK+, the electrode potentials of the potassium selective electrode relative to the silver chloride electrode were measured and the following data were obtained:
сK+, mol/l 1.0∙10-1 1.0∙10-2 1.0∙10-3 1.0∙10-4
E, mV 100 46.0 -7.00 -60.0
Based on these data, a calibration curve was built in the coordinates E - pсK+.
A 0.2000 g sample containing potassium was dissolved in water and the volume was adjusted to 250.0 ml. Then measured the electrode potential of the potassium selective electrode in the resulting solution Ex =3.0 mV. Calculate the mass fraction (%) of potassium in the sample.

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