10-mL aliquots of a natural water sample were pipetted into 50.00 mL volumetric flasks. Exactly 0.00, 5.00, 10.00, 15.00, and 20.00 mL of a standard solution containing 11.1 ppm of Fe3 were added to each followed by excess of thiocyanate to give the red complex Fe(SCN)2 . After dilution to volume absorbances for the five solutions were measured and they are 0.240, 0.437, 0.621, 0.809, 1.009. What is the concentration of Fe3 in the sample