Question

A standard iron solution was prepared by dissolving 0.0100 g of pure iron in acid then transferring to a 50.00 mL volumetric flask with orthophenanthroline as a complexing agent. At a wavelength of 540 nm, the absorbance of the standard was 0.239 in a 1.00 cm cuvette. A 0.149 g sample of an iron ore was crushed and digested in 5 mL of concentrated acid. The digested sample was then transferred to a 10.00 mL volumetric flask and diluted to the mark with water. A 1.00 mL aliquot of the sample was transferred to a 25.00 mL volumetric flask, and diluted to the mark with water. A portion of the diluted sample was then transferred to a 1.00 cm cuvette and measured at a wavelength of 540 nm. The resulting absorbance was 0.212. Determine the % w/w of Fe in the iron ore. %w/w

Answer 1

The concentration of the standard solution is calculated:

Cst = 0.01 g / 50 mL * (1 mol / 55.8 g) * (1000 mL / 1 L) = 0.0036 M

Through the law of lambert beer, the molar absorptivity is calculated:

E = A / C * L = 0.239 / 0.036 M * 1cm = 66.4 M-1 cm-1

The molar concentration of the unknown solution is calculated:

C = A / E * L = 0.212 / 66.4 * 1 = 0.0032 M

The molar concentration in the first dilution is calculated:

Initial C = 0.0032 M * 25 mL / 1 mL = 0.08 M

The mass of Fe is calculated:

m Fe = 0.08 M * 0.01 L * 55.8 g / mol = 0.0446 g

The percentage is calculated:

% Fe = 0.0446 * 100 / 0.149 = 30%

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