Question

Why is a large excess of Fe³⁺ added when preparing the standard solution. But you not add a large excess of Fe³⁺ when preparing the equilibrium solution.

Fe^3+ + SCN^- = FeSCN^2+

Answer 1

Fe3+ + SCN- = FeSCN2+

​​​​A very large concentration of Fe3+ is added to a small concentration (known) of SCN- ( shown as [SCN-]0) to prepare standard solution. The [Fe3+] taken is 100 times than [Fe3+] in the equilibrium mixtures.

To understand the reason of that, we have to consider Le Chatelier's principle, this high concentration of reactant forces the reaction far toward formation of products, using up nearly 100% of the
[SCN-]0.

Thus in the prepared standard solution [FeSCN2+] is assumed to be equal to [SCN-]0 initially taken.