Equilibria can be treated mathematically with the equilibrium constant, K. For the reaction Fe^3+ + SCN − equilibrium reaction arrow FeSCN^2+, the equilibrium constant expression is [FeSCN^2+ ] K = [Fe^3+ ] · [SCN − ] where, for example, [Fe3+ ] is the molar concentration (mol Fe^3+ / L solution) present in an equilibrium mixture. At some temperature, a chemist found the following equilibrium concentrations. [Fe3+ ] = 8.17 ✕ 10^−3 M, [SCN − ] = 8.60 ✕ 10^−3 M, [FeSCN^2+ ] = 6.25 ✕ 10^−2 M a. What is value of K?
Suppose that some FeSCN^2+ is added to the above solution to shift the equilibrium. When equilibrium is re-established, the following concentrations are found.
[Fe3+ ] = 8.34 ✕ 10^−3 M, [SCN− ] = 8.77 ✕ 10^−3 M

b. What is the concentration of FeSCN2+ in the new equilibrium mixture?