chem

... Experiment: Study of a complex ion equilibrium Aim: To determine the formula and formation constant of iron (III) thiocyanate ion by sprectrophotometry Procedure: Please refer to the lab manual Result: solution number 1 2 3 4 5 6 7 8 9 10 volume of solution A used /mL 0 1 3 5 7 9 11 13 15 16 i = [A]initial / M 0 0.00012 0.00036 0.0006 0.00084 0.00108 0.00132 0.00156 0.0018 0.00192 Total absorbance 0 0.019 0.057 0.075 0.084 0.087 0.074 0.057 0.023 0.005 Absorbance due to Fe3+ (A10 . Vp/V10) 0 0.0003125 0.0009375 0.0015625 0.0021875 0.0028125 0.0034375 0.0040625 0.0046875 0.005 Absorbance due to the complex ion ABn 0 0.0186875 0.0560625 0.0734375 0.0818125 0.0841875 0.0705625 0.0529375 0.0183125 0 mole fraction of A 0 0.0625 0.1875 0.3125 0.4375 0.5625 0.6875 0.8125 0.9375 1 y = [ABn] / M 0 0.00012 0.00036 0.0004716 0.0005254 0.0005406 0.0004531 0.0003399 0.0001176 0 value of K - - 3.893E+12 4327.8564 3010.2702 3347.4912 3558.3531 13884.485 29025.856 - Calculations: Concentration of solution A = [A]in = 0.003M Concentration of solution B = [B]in = 0.003M Let the initial concentration of A be i, the initial concentration of B be c-i. the concentration of ABn be y when equilibrium is established A + nB › ABn [ABn]eq = y [A]eq = i-y [B]eq = c-i-ny Equilibrium constant K = y/ [(i-y)(c-i-ny)n] Taking solution number 3 for full calculation Initial concentration of A (i) = [A]soln 3 × vol of A in soln. 3 / total volume of soln = 0.003 x 3/25 = 3.6 x 10-4 M Total absorbance for solution

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