7. In standard solutions of potassium salt with a concentration of SC+, the electrode potentials of the potassium selective electrode relative to the silver chloride electrode were measured and the following data were obtained:
SC+, mol∙l–1 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, build a calibration graph in the coordinates E - pSK+.
A 0.2000 g sample containing potassium was dissolved in water, and the volume was adjusted to V (ml). Then the electrode potential of the potassium selective electrode Ex in the resulting solution was measured:
Calculate the mass fraction of potassium w (%) in the sample.

Option 7
V, ml 300
Ex, mV 2

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