You wish to measure the iron content of the well water on the new property you a

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 Fe^3+ solution with a concentration of 7.38 times 10^-4 M. You treat 10.0 mL of this reference with HNO_3 and excess KSCN to form a red complex, and dilute the reference to 45.0 mL. The diluted reference is placed in a cell with a 1.00-cm light path. You then take 20.0 mL of the well water, treat with HNO_3 and excess KSCN, and dilute to 100.0 mL. This diluted sample is placed in a variable path length cell. The absorbance of the reference and the sample solutions match when the path length is 4.03 cm. What is the concentration of iron in the well water? For each solution, the zero is set with a blank.

Explanation / Answer

Cref = 7.38 x 10^-4 x 10 / 45

       = 1.64 x 10^-4 M

A = e b C

Csample = C ref x b ref / b sample

               = 1.64 x 10^-4 x 1 / 4.03

               = 4.07 x 10^-5 M

[ Fe+3 ]well water = 4.07 x 10^-5 x 100 / 20

                             = 2.04 x 10^-4 M

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