$($c$)$ The chromium concentration in an aqueous sample was determined by pipetting $10\mathrm{.}0c{m}^3$ of the unknown into each of five $50c{m}^3$ volumetric flasks. Various volumes of a standard solution containing $5ppm$ chromium were added to the flasks following which the solutions were diluted to $50c{m}^3.$ Using the data in the table below, plot an appropriate graph and calculate the chromium concentration in the unknown solution.

$\backslash$table$[[$Unknown $\frac{?}{c{m}^3},$Standard $\frac{?}{c{m}^3},$Absorbance$],[10,0,0\mathrm{.}201],[10,10,0\mathrm{.}292],[10,20,0\mathrm{.}378],[10,30,0\mathrm{.}467],[10,40,0\mathrm{.}554]]$