Question

Part A of lab: pour 30 ml of 0.150 M Fe(NO3)3 into a 50 ml beaker and pipette 10 ml of this solution into a 25 ml volumetric flask. This is the standard reagent blank

Part B of lab: pour 30 ml of 1.50*10^-3 M Fe(NO3)3 into a 25 ml volumetric flask. This is the reagent blank.

Why is a large excess of Fe3+ added to make the standard solution, and why do you NOT add a large excess of Fe3+ when preparing the equilibrium solution??

Answer 1

To make the Standard Solution, it must be ensured that entire Reaction leads to completion, thereby ensuring maximum Concentration of Fe(SCN)²⁺. Hence, a large excess of Fe^{​​​​​3+} Solution is added, to shift the Equilibrium towards the Product.

In preparing the Equilibrium Solution, if large excess of Fe³⁺ Solution is added, the System will not attain Equilibrium, it will shift the Equilibrium towards Fe(SCN)²⁺ . And thus The Equilibrium Constant couldn't be measured. Thus, for the attainment of the Equilibrium, large excess of Fe^{​​​​​3+} Solution is to be avoided.