21. Solving a chemistry problem

1. A standard solution of calcium was prepared by dissolving an accurate sample of CaCO3 weighing 1.1000 g in HCl. The solution was made up to the mark with water in a 100 ml volumetric flask. From the resulting solution, by successive dilution in volumetric flasks of each previous solution by a factor of 10, a series of more diluted standard solutions was prepared and the electrode potential of the calcium selective electrode was measured in them with respect to the silver chloride electrode:
С(Ca2 ), mol/l n*10-1 n*10-2 n*103 n*10-4 n*10-5

E, mV 58 39 18 -20 -40

Based on these data, a calibration curve was built in the coordinates E-pC(Ca2). The analyzed solution was placed in a volumetric flask with a capacity of 100 ml and made up to the mark with water. An aliquot of 5 ml of the resulting solution was transferred into a 50 ml flask, 5 ml of 1M KCl solution was added there, and the volume was brought to the mark. The solution was placed in the cell and the potential of the calcium selective electrode Ex was measured. it turned out to be -2mV. Determine the mass of calcium in the test solution in grams.