Question

The Beer’s Law calibration curve for this experiment was obtained at the appropriate wavelength and gave the following linear regression equation:
Absorbance = 4898 (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 0.30. 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)

Answer 1

The Beer’s Law calibration curve for this experiment was obtained at the appropriate wavelength and gave the following linear regression equation:
Absorbance = 4898 (Concentration) + 0.055

plot Abs. vs . Conc.(c)

Slope of this plot,    Slope = ε *l

ε = molar absorptivity , l = path length

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 0.30.

[Fe²⁺] = 5.0 mL * 0.0020 M /10 mL (total Volume) = 0.001 M

[HSCN] = 5.0 mL * 0.0020 M /10 mL (total Volume) = 0.001 M

Since , [Fe(SCN)]²⁺ is only absorbing species.

from , regression equation :

0.30 = 4898*C + 0.055

Conc. : 5*10^-5 M

[Fe(SCN)]²⁺ = 5*10^-5 M

[H+] = 0.50 M

For reaction , Fe³⁺(aq) + HSCN(aq) ⇌ FeSCN²⁺(aq) + H⁺(aq)

Keq = [H+] [Fe(SCN)]²⁺ / [Fe²⁺] [HSCN]

at eq. [Fe²⁺] = [HSCN] = 0.001 M - 5*10^-5 M = 0.00095 M

Keq = [0.50] [5*10^-5 ] / [0.00095] [0.00095] = 27.7