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 6.35 Ă— 10-4 M. You treat 10.0 mL of this reference with HNO3 and excess KSCN to form a red complex, and dilute the reference to 55.0 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 4.40 cm. What is the concentration of iron in the well water? For each solution, the zero is set with a blank.

Answer 1

Based on Beer-Lambert's Law,

A = εcl ------(1)

where A = absorbance

ε = molar absorptivity

c = concentration

l = path length

Step 1: Calculate the concentration of the diluted Fe3+ standard

Use:

V1M1 = V2M2

M2 = V1M1/V2 = 10 ml*6.35*10⁻⁴M/55 ml = 1.154*10⁻⁴ M

Step 2 : Calculate the concentration of the sample solution

Based on equation (1) we have:

A(Fe3+) = ε(1.154*10⁻⁴)(1)

A(sample) = ε(C)(4.4)

It is given that the absorbances match under the given path length conditions, i.e.

ε(1.154*10⁻⁴)(1) = ε(C)(4.4)

C = 0.262*10⁻⁴ M

This is the concentration of Fe3+ in 100 ml of well water sample

Step 3: Calculate the concentration of Fe3+ in the original sample

Use V1M1 = V2M2

M1 = V2M2/V1 = 100 ml * 0.262*10⁻⁴ M/35 ml = 7.49*10⁻⁵M

Ans: Concentration of F3+ in the well water sample is 7.49*10⁻⁵M