iron (iii) Fe3+ ions, form a colored complex with thiocyanate SCN-, that absorbs

Question

iron (iii) Fe3+ ions, form a colored complex with thiocyanate SCN-, that absorbs at 472 nm in the UV- visible spectral range. in a lab experiment, an iron vitamin table was digested in acid and diluted in acid and diluted to 250ml. A 5.0 ml aliquot of this sample when treated with a solution of potassium thiocyanate and diluted to 100ml produced an absorbance of 1.700 when a 1.00cm cuvette was used. use the graph below to determine the molar concentration of the vitamin stock solution and the number of miligrams of iron in this vitamin tablet

Explanation / Answer

The linear equation of graph is

y = 7000x + 3 X 10^-5

where x represents concentration

y represents absorbance

given absrobance = y= 1.7

1.7 = 7000x + 0.000003

x = 2.43 X 10^-4 molar

This is the concentration when 5mL stock was diluted to 100mL

So we may use the following relation

M1V1 = M2V2

M1 X 5 = 100 X 2.43 X 10^-4

M1 = molarity of stock solution = 48.6 X 10^-4 molar

Molarity of Fe+3 = 48.6 X 10^-4

Moles = Molarity X volume = 48.6 X 10^-4 X 0.25 L = 12.15 X 10^-4 moles

So mass of Fe+3 = Moles X atomic weight = 12.15 X 10^-4 X 55.8

Mass = 677.97 X 10^-4 grams = 67.797 mg

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