Question

You wish to measure the iron content of the well water on the new property you are about to buy. You prepare a reference standard Fe3 solution with a concentration of 6.75 × 10-4 M. You treat 12.0 mL of this reference with HNO3 and excess KSCN to form a red complex, and dilute the reference to 40.0 mL. The diluted reference is placed in a cell with a 1.00-cm light path. You then take 35.0 mL of the well water, treat with HNO3 and excess KSCN, and dilute to 100.0 mL. This diluted sample is placed in a variable pathlength cell. The absorbance of the reference and the sample solutions match when the pathlength is 1.59 cm. What is the concentration of iron in the well water? For each solution, the zero is set with a blank.

Answer 1

Cref = 6.75 x 10^-4 x 12 / 40

       = 2.025 x 10^-4 M

A = e b C

Csample = C ref x b ref / b sample

               = 2.025 x 10^-4 x 1 / 1.59

               = 1.27 x 10^-4 M

[ Fe+3 ]well water = 1.27 x 10^-4 x 100 / 35

                             = 3.64 x 10^-4 M