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The amount of iron in steel can be determined by titration with dichromate. First, the steel is dissolved in hydrochloric acid to oxidize all the iron to Fe 2 + . An indicator is added that will react with excess dichromate. Then, potassium dichromate is added to the solution until the end point is reached. Cr 2 O 2 − 7 + 6 Fe 2 + + 14 H + ⟶ 2 Cr 3 + + 6 Fe 3 + + 7 H 2 O For the analysis of 2.00 g of steel, the titration required 21.20 mL of 0.1800 M K 2 Cr 2 O 7 . What was the mass percentage of iron in this steel sample?

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Answer:

% Fe=  63.9

Explanation:

The amount of iron that was titrated can be calculated as follows:

mol K₂Cr₂O₇ =  0.02120 L x 0.180 M = 3.82 x 10⁻³

mol Fe titrated = 3.78 mol Cr₂O₇²⁻  x 10⁻³ x 6 mol Fe²⁺/mol Cr₂O₇²⁻

                         = 2.29 x 10⁻²

mass Fe = 2.29 x 10⁻² mol  Fe²⁺ x 55.845 g/mol Fe²⁺ = 1.28 g Fe

% Fe = 1.28 g  Fe / 2.00 g steel  x 100 g steel = 63.9 %

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