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 7.39 × 10-4 M. You treat 13.0 mL of this reference with HNO3 and excess KSCN to form a red complex, and dilute the reference to 50.0 mL. The diluted reference is placed in a cell with a 1.00-cm light path. You then take 10.0 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 path length is 3.51 cm.

What is the concentration of iron in the well water? For each solution, the zero is set with a blank.

Answer 1

Conc of standard reference solution = 7.39*10^-4 M

Conc of diluted reference solution taken in the cuvette, C₁ = (7.39*10^-4)/(50/13) = 1.9214*10^-4 M

Assume that conc of iron in well water is 'x' M

Conc of diluted well water taken for measurement in cuvette, C₂ = x/(100/10) = x/10

According to Beer Lambert's law, we have:

A = e*l*C

So, using the relation:

l₁*C₁ = l₂*C₂

Here the subscript 1 is for standard and 2 is for well water.

Putting values:

1*1.9214*10^-4 = 3.51*(x/10)

Solving we get:

x = 5.474*10^-4 M

Hope this helps !