Iron(II) ion salts, such as FeSO_4 7 H_2O, react with the organic compound to fo

Question

Iron(II) ion salts, such as FeSO_4 7 H_2O, react with the organic compound to form red coordination compounds with formulas such as Such compounds have an analytical wavelength of 510 nm. A standard solution containing Fe(phen) was made by dissolving 0.0139 g of 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 concentration has of 64.3% at 510 nm. What is the molar concentration of in the unknown solution?

Explanation / Answer

(1) Absorbance = 2 - log(%T)

Standard Fe(Phen)nSO4 solution concentration = 0.0139 g/278.05 g/mol x 0.5 L = 0.0001 M

Absorbance of standard solution = 2 - log(36.5) = 0.44

Absorbance of unknown solution = 2 - log(64.3) = 0.19

Concentration of unknown solution = 0.19 x 0.0001/0.44 = 4.32 x 10^-5 M

(2) grams of Fe2+ in the solution = 4.32 x 10^-5 mol/L x 55.84 g/mol = 2.41 mg/L

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