2. In standard solutions of potassium salt with concentration C(K+), the electrode potentials of the potassium-selective electrode relative to the silver chloride electrode were measured:
C (K+), mol/l 1× 10-1 1× 10-2 1× 10-3 1× 10-4
E, mV 100 46.0 -7.00 -60.0

Based on these data, a calibration graph was constructed in E–pC (K+) coordinates.
A sample weighing 0.2000 g containing potassium was dissolved in water, and the volume was brought to 25.00 ml. Then we measured the electrode potential Ex = 34.0
Calculate the mass fraction (%) of K in the sample.

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