10 mL aliquots of natural water were pipetted into 50 mL flasks. Exactly 0.0, 0.5. 10.0, 15.0, and 20.0 mL of a standard solution containing 11.1 ppm of Fe were added to each, followed by an excess of thiocyanate ion to give the red complex Fe(SCN). After dilution to volume, the instrumental signal S for each of the five solutions, measured with colorimeter, was found to be: 0.240, 0.437, 0.621, 0.809, and 1.009, respectively. What was the concentration of Fe in the water sample?