A 2.893 g unknown sample contains iron in the form of the Fe^+2 ion. When the sa

Question

A 2.893 g unknown sample contains iron in the form of the Fe^+2 ion. When the sample is dissolved and an a Redox titration is performed, 28.45 mL of 0.01971 M KMNO_4 is required to arrive at the red colored end point. The reaction is shown for the iron reacting with permanganate. The number of grams of Fe^+2 ion determined is grams Fe^+2. (answer has four sig. figs.) Find the absorbance of a 0.00240 M solution of a substance with a molar absorptivity of 1.00 Times 10^2 M^-1 cm - in a cell with a 2.00 cm pathlength.

Explanation / Answer

Redox reaction between KMnO4 and Fe2+ in acidic medium can be written as,

MnO4- (aq.) + 5 Fe2+(aq.) -----------> Mn2+ (aq.) + Fe3+ (aq.)

Moles of KMnO4 used = 0.01971 * 28.45 / 1000 = 0.000561 mol

From the balanced equation,

1 mol KMnO4 = 5 mol Fe2+

Then, 0.000561 mol KMnO4 = 5 * 0.000561 = 0.0028 mol of Fe2+

Mass of Fe2+ = 0.0028 * 56 = 0.1571 g.

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