Problem #$1$: A $10\mathrm{.}00mL$ aliquots of a natural water sample were pipetted into $50\mathrm{.}00mL$ volumetric flasks. Exactly $0\mathrm{.}00,5\mathrm{.}00,10\mathrm{.}00,15\mathrm{.}00,$ and $20\mathrm{.}00mL$ of a standard solution containing $11\mathrm{.}1ppm$ of $F{e}^{3+}$ were added to each, followed by an excess of thiocyanate ion to give the red complex $Fe(SCN{)}^{2+}.$ After dilution to volume, absorbances for the five solutions, measured with a photometer equipped with a green filter, were found to be $0\mathrm{.}240,0\mathrm{.}437,0\mathrm{.}621,0\mathrm{.}809,$ and $1\mathrm{.}009$ respectively. What was the concentration of $F{e}^{3+}$ in the water sample?