Q6. (5 points) Calamine, which is used for relief of skin irritations, is a mixt

Question

Q6. (5 points) Calamine, which is used for relief of skin irritations, is a mixture of zinc and iron oxides. A 1.022 g sample of dried calamine was dissolved in acid and diluted to 250.0 mL. Potassium fluoride was added to a 10.0 mL aliquot of the diluted solution to mask the iron: after suitable adjustment of the pH, Zn2 consumed 38.71 mL of 0.01294 M EDTA. A second 50.0 mL aliquot was suitably buffered and titrated with 2.40 mL of 0.002727 M ZnY solution: znY Fe Zn Fe Calculate the percentage of Zno and Fe2O3 in the sample. Dept. of Chemistry, UOB Page

Explanation / Answer

Moles of EDTA consumed by Zn2+(aq) = MxV(L) = 0.01294 mol/L x 0.03871 L = 5.01x10-4 mol

Hence moles of Zn2+(aq) in 10 mL of the diluted calamine solution =  5.01x10-4 mol

=> moles of Zn2+(aq) in 250.0 mL of the diluted solution(containing 1.022 g calamine)

= (5.01x10-4 mol / 10 mL) x 250 mL

= 0.01252 mol Zn2+(aq)

Hence moles of ZnO in 1.022 g calamine sample = moles of Zn2+(aq) = 0.01252 mol ZnO.

molecular mass of ZnO = 81.38 g/mol

Hence mass of ZnO in the calamine sample = 0.01252 mol x 81.38 g/mol = 1.019 g

Hence percentage of ZnO in the sample = (1.019 g / 1.022 g)x100 = 99.7 % ZnO (answer)

and percentage of Fe2O3 in the sample = 100 - 99.7 = 0.3 % (answer)

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