Potassium dichromate in acidic solution is used to titrate a solution of iron(II

Question

Potassium dichromate in acidic solution is used to titrate a solution of iron(II) ions, with which it reacts according to Cr_2O_7^2- (aq) + 6 Fe^2+ (aq) + 14H_3O^+ (aq) rightarrow 2Cr^3+ (aq) + 6Fe^3+ (aq) + 21 H_2O (l) A potassium dichromate solution is prepared by dissolving 5.134 g of K_2Cr_2O_7 in water and diluting to a total volume of 1.000 L. A total of 34.26 mL of this solution is required to reach the end point in a titration of a 500.0-mL sample containing Fe^2+ (aq). Determine the concentration of Fe^2+ in the original solution. From Principles of Modern Chemistry (6^th ed.) by Oxtoby, Gillis, and Campion.

Explanation / Answer

1 molle of K2Cr2o7 requires 6 moles of Fe+2.

Moles of K2Cr2O7 in 5.134 gm = mass/Molecular weight =5.134/294=0.0174 moles

1 liter =1000ml of solution contains 0.0174 moles

34.26 ml solution contains34.26*0.0174/1000 = 0.00596 moles

moles of Fe+2 required =6*0.000596 =0.003577 moles

This is tirated with 500ml of Fe+2 solution, Concentration of Fe+2= 0.003577/0.5 moles/ L =0.007153 moles/L

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