The Beer’s Law calibration curve for this experiment was obtained at the appropr

Question

The Beer’s Law calibration curve for this experiment was obtained at the appropriate wavelength and gave the following linear regression equation: Absorbance = 4137 (Concentration) + 0.055 Suppose 5.0 mL of 0.0020 M Fe(NO3)3 dissolved in 0.50 M HNO3 are mixed with 5.0 mL of 0.0020 M HSCN dissolved in 0.50 M HNO3, and the absorbance of the solution is measured to be 0.23. What is the equilibrium constant for the following reaction? Assume that the H+ concentration stays constant at 0.50 M. Fe3+(aq) + HSCN(aq) FeSCN2+(aq) + H+(aq)

Explanation / Answer

molar absorptivity = 4137 M-1.cm-1

[FeSCN]2+(eq) = 0.23/4137 = 5.56 x 10^-5 M

[Fe3+](eq) = (0.002 x 0.005/11) - 5.56 x 10^-5 = 8.53 x 10^-4 M

[SCN-] (eq) = (0.002 x 5/11) - 5.56 x 10^-5 = 8.53 x 10^-4 M

Equilibrium constant K = [FeSCN]2+(eq)/[Fe3+(eq)[SCN-](eq)

K = (5.56 x 10^-5)/(8.53 x 10^-4)^2 = 76.41

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