Question

The reaction of cobalt(II) nitrate and potassium thiocyanate produces a coloured complex, [Co(SCN)4]2-(aq). The initial concentrations of the reactants after mixing (before any reaction) were 0.250 mol/L Co(NO3)2(aq) and 0.500 mol/L KSCN (aq). After 20 minutes, the absorbance of the solution was 0.825. The complex, [Co(SCN)4]2-, is the only absorbing species (ε b = 9.29 (mol/L)-1). Determine the concentration of Co2+ at equilibrium.

Answer 1

Co(NO3)2 + 4 KSCN $\rightarrow$ [ Co(SCN)4]-2 (aq)

[Co(SCN)4]-2 $\rightarrow$ Co+2 + 4 SCN- ----- (A)

Given: Aborbance of [Co(SCN)4]2- is 0.825

€ b = 9.29 (mol/L)-1

As, A = € c b -------(1)

Where € is molar Absorpitivity

c is the concentration

b is the length

Putting the value in (1)

c = A/€ b

= 0.825/9.29

= 0.088M of [ Co(SCN)4]-2

So, according to above (A) reaction

Concentration of Co+2 = 0.088 M