Question

Calculate pFe2+ at each of the points in the titration of 25.00 mL of 0.02219 M Fe2+ by 0.03689 M EDTA at a pH of 7.00. The values for log K and Qyt can be found in the chempendix. 11.50 mL pFe2+ = the equivalence point, V. pFe2+ = 19.50 mL pFe2+ =

Answer 1

Conc. of Fe+2 solution = 0.02219 M. Conc.of EDTA = 0.3689 M Volume of Fe+2 solution = 25.00 mL log Kf = 14.30 Formation constant of [Fe(EDTA)1-2 = 1014.30 = 2.00x1014 At pH =7, the fraction of EDTA that exists Y-4 is Ay+ = 3.8x104 So the Conditional Formation constant = K = 3.8x10+ x 2.00 x 1024 = 7.58x1010 (a) 11.50 ml mmols of Fe+2 = M*V = 0.02219 M* 25.00 mL = 0.55475 mmol Number of moles of EDTA = 0.03689 M * 11.50 mL = 0.424235 mmol This is before equivalence point. Net moles of Fe+2 = 0.55475 -0.424235 = 0.130515 mmol Total volume of the solution = 25.00 + 11.50 = 36.50 mL Conc. of Fe+2 = moles/volume = 0.130515 mmol/36.50 mL = 0.003576 M pFe+2 = -log[Fe+2] = -log(0.003576) = 2.447 = 2.45

b) At equivalence point: The number of moles of Fe+2 is equal to the number of moles of EDTA So the number of moles of EDTA at equivalence point = 0.55475 mmol Volume of EDTA required to reach equivalence point = moles/molarity = 0.55475 mmol/ 0.03689 M = 15.04 mL initial moles of Fe+2 0.55475 mmol -=0.01386M Total Volume (25.00+15.04) mL - =- At equivalence point, all Fe+2 is converted to [FeY-2] complex. So the concentration of Fe+2 is determined by the dissociation of [FeY-2] complex. K' Fe+2 + y4A Fer-2 Initial 0 0 0.01386 Change + x + x -X Equilibrium x x 0.01386 - X ? = K 7.58 x 1010 = 1. 10 = 1.83x10-13 FeY-21 0.01386-X 0.01386 ^, *[Fe*?][Y] (ix is small) x.x L2_0.01386 0.01386 K x = 4.27x10-7 [Fe*?] = 4.27x10-1 => pFe*? = -log[Fe*?]=- log (4.27x10^) = 6.3691 = 16.37

c) Addition of 19.50 mL of EDTA mmols of Fe+2 = M*V = 0.02219 M* 25.00 mL = 0.55475 mmol mmols of EDTA = M*V = 0.03689 M * 19.50 mL = 0.719355 mmol At this point EDTA present in excess. No. of moles of EDTA present in excess = 0.719355 – 0.55475 = 0.164605 mmol the concentration of Fe+2 is determined by the dissociation of [FeY-2] complex. moles of Fe+2 [Fer-?]=mo 0.55475 mmol = 0.01247M Total Volume (25+19.50) mL no.of moles of EDTA-no. of moles of Fe-2 [EDTA] = no.of Total Volume _0.164605 mmol :=0.003699 M 44.50 mL (Fey-21 k; +[Fe][Y*] >[Fe+21= [Fey?) - 0.01247 -= 4.44x10-11 "K,x[Y"] 7.58x1060 x0.003699 [Fe*?]=4.44*10-"M =pFe*? =- logſ Fet? 7 =- log (4.44x10-11)=10.3521 = 10.35