You make a set of standards (solution of known concentration of the molecule/ions you want to determine), run them through the same procedure you will use on the unknowns, then measure the absorbance of each. Plot A on the y axis and mg/mL on the x axis. That is your calibration curve. Then you run the unknowns, measure A values, use the calibration curve to convert A to mg/mL and report your results.
