119. Solving a chemistry problem

The analyzed sample of Mohr´s salt weighing 0.3922 g was dissolved in sulfuric acid with a molar concentration of 0.5 mol/l, potentiometric titration of iron(II) was carried out with a standard solution of cerium(IV) sulfate with a molar concentration equivalent of 0.0500 mol/l and obtained the following results (V is the volume of added titrant, E is the EMF of the circuit):
V, ml 10.00 18.00 19.80 20.00 20.20 22.00 30.00
E, mV 771 830 889 1110 1332 1391 1453
Determine the mass fraction of iron(II) in the analyzed sample by graphic and calculation methods. Answer: 14.24%.