Question

An unknown solution containing iron at a concentration between 10 and 50 mg/L is now to be analyzed. 5 mL of this unknown solution is diluted to 100 mL with the appropriate reagents to form the ferrous - phenanthroline complex. The diluted solution has an absorption of 0.614. A. What is the concentration in (mg/L) of ferrous ion in the diluted solution? B. What is the concentration of iron in the unknown solution (before it was diluted), in mg L-1 ?

Answer 1

we known that Absorbance A= ebC

where A= 0.614, e= moalr extinction coefficient of [(C2H8N2)3Fe]⁺² 11,100 L/molcm. and b= path length= 1cm (assumed)

0.614=11100*1C

C= 0.614/11000=5.58*10-5M

atomic weight of iron =56

Mass of iron in 1 L = 5.58*10^-5*56 *1000mm/L=3.125 mg

moles of iron in 100ml; = 5.58*10-5*100/1000= 5.58*10-6

this many moles are there in 5ml of unknown solution

Do concentration = 5.58*10-6*1000/5 moles/L= 5.8*10-6*56*200*1000 mg/L=62.496 mg/L

so this is not the range in which concentration is measured, assume a path length of 1.5cm

and repeat the calculations

C now = 0.614/(1.5*11000)=3.72*0-5 Moles/L

Concentration expressed in mg/L= 3.72*10-5*56*1000mg/L=2.08 mg/L

moles of iron in 100ml =3.72*10-5*100/1000= 3.72*10-6 moles

this many moles are there in 5ml of unknown solution

hence concentration= 3.72*10-6*1000/5= 0.000744 moles/L

expressed in terms of mg/L= 0.000744*56*1000=41.664 mg/L.

This is falling into the range to be measured.