Question

For preparing a set of standard solutions of FeNCS2+, the equilibrium molar concentration of FeNCS2+ is assumed to equal the initial molar concentration of the (SCN-) in the reaction mixture. why is this assumption valid?

Answer 1

This assumption is valid because the reaction between Fe3+ and SCN- to form FeNCS2+ (also written as Fe(SCN)2+) has a very large equilibrium constant (K). This means that the reaction strongly favors the formation of the FeNCS2+ complex, and nearly all of the SCN- ion will react with Fe3+ to form the complex.

In this case, Fe3+(aq) + 2 SCN-(aq) <=> Fe(SCN)2+(aq)

Since the equilibrium constant is very large, we can assume that the reaction proceeds almost to completion. Therefore, the initial molar concentration of the SCN- ion would be the same as the molar concentration of FeNCS2+ in the equilibrium state. This assumption makes it easier to calculate and prepare the standard solutions required for the experiment, as well as simplifying the equilibrium calculations.