Iron(II) ion salts, such as FeSO_4 middot 7 H_2O, react with the organic compoun

Question

Iron(II) ion salts, such as FeSO_4 middot 7 H_2O, react with the organic compound ortho-phenanthroline (phen) to form red coordination compounds with formulas such as Fe(phen)_n SO_4. Such compounds have an analytical wavelength of 510 nm. A standard solution containing Fe(phen)_n SO_4 was made by dissolving 0.0139 g of FeSO_4 middot 7H_2O, with a molar mass of 278.05 g mol^-1, in 500.0 mL of water containing excess phen. The solution has a %T of 36.5% at 510 nm. A 500.0-mL solution with unknown Fe(phen)_n SO_4 concentration has a %T of 64.3% at 510 nm. What is the molar concentration of Fe(phen)_n SO_4 in the unknown solution?

Explanation / Answer

Molarity of sample c1 = mass/ molar mass x 1/volume in Litres

= 0.0139 g/ 278.05 g/mol x 1/ 0.5 L

= 10-4 M

Given that T1 = 36.5 %

T2 = 64.3 %

c2 = ?

Beer's law is   

A = ecl

A = absorbance

e = molar extinction coefficient

c = concentration

l = path length

Relation between Absorbance and Transmittance is

A = 2 - log %T

ecl = 2 - log %T

Hence,

ec1l / ec2l = 2 - log %T1 / 2 - log %T2

c1/c2 = 2 - log %T1 / 2 - log %T2

10-4 M/ c2 =  (2 - log 36.5 ) / (2 - log 64.3)

c2 = 4.4 x10-5 M

Therefore,

Molar concentration of Fe(phen)nSO4 in the unknown sample = 4.4 x10-5 M

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