The answers are b) 2.33E-7 mol Fe(III) c) 5.83E-5 M. I dont know what a is and i

Question

The answers are b) 2.33E-7 mol Fe(III) c) 5.83E-5 M. I dont know what a is and i do not understand how to get these answers.

Spectrophotometric Titrations 18-26. A 2.00-mL solution of apotransferrin was titrated as illus- trated in Figure 18-11. It required 163 ML of 1.43 mM ferric nitrilo. triacetate to reach the end point. (a) Why does the slope of the absorbance versus volume graph change abruptly at the equivalence point? (b) How many moles of Fe ferric nitrilotriacetate) were required to reach the end point? (c) Each apotransferrin molecule binds two ferric ions. Find the molar concentration of apotransferrin in the 2.00-mL solution.

Explanation / Answer

a) Fe(III) solution is pale brown in colour. When we begin the titration, all the Fe(III) ions we add will bind to apo-transferrin and the net solution will be colourless as there are no free Fe(III) ions in the solution. Note, we know that each apo-transferrin can bind to two Fe(III) ions. The solution will remain colourless till we reach the equivalence point. After this point, there will be free Fe(III) ions in the solution thus making the solution coloured. Absorbance measurements are sensitive to change in colour. Hence due to this colour change, we see an abrupt change in the slope after equivalence point.

b) Number of moles of Fe(III) = molarity of Fe(III) solution X Volume of Fe(III) solution

= 1.43 X 10-3 mol L-1 X 163 X 10-6 L    

Note 1 L = 10-6 L and 1 mM = 10-3 M = 10-3 mol L-1

Number of moles of Fe(III) = 233 X 10-9 mol

= 2.33 X 10-7 mol

c) It is given that each apo-transferrin can bind to two Fe(III) ions

Therefore 1 mole of apo-transferrin can bind to 2 mol Fe(III) ions

Number of moles of apo-transferrin = ½ Number of moles of Fe(III)

= ½ X 2.33 X 10-7 mol

= 1.17 X 10-7 mol

Concentration of apo-transferrin = Number of moles of apo-transferrin / Volume of apo-transferrin

= 1.17 X 10-7 mol / 2.0 X 10-3 L

Note 1 mL = 10-3 L

Concentration of apo-transferrin = 0.585 X 10-4 mol L-1

= 5.85 X 10-5 mol L-1

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