5. Solving a chemistry problem

9. During the potentiometric determination of calcium in a solution by the method of a calibration curve in standard solutions of calcium salts with a concentration c(Ca), the electromotive force E between the indicator calcium-selective electrode and the calomel reference electrode was measured:
c(Ca2), mol/l 0.0001 0.001 0.01 0.1
E,mV -27 -1 21 47
What is the molar concentration of calcium ions in the analyzed solution if the electromotive force of the circuit in this solution turned out to be 12 mV.