Question

In the calibration solutions, the very large excess of Fe3+ ion compared to the amount of NCS- ion used to prepare the the mixtures allows us to make what assumption about the concentration of product in the mixtures? Question 8 options:

[Fe(NCS)2+]equilibrium = [NCS-]equilibrium [Fe(NCS)2+]equilibrium = [Fe3+]initial [Fe(NCS)2+]initial = [Fe3+]initial [Fe(NCS)2+]equilibrium = [Fe3+]equilibrium [Fe(NCS)2+]equilibrium = [NCS-]initial [Fe(NCS)2+]initial = [NCS-]initial

Answer 1

Ans:

The standard solution contains a very large concentration of Fe³⁺ ([Fe³⁺]_i) and a small initial concentration of SCN^– ([SCN⁻]_i). (The [Fe³⁺] in the standard is 100 times more than [Fe³⁺] in the trials.) The high [Fe³⁺] forces the reaction far to the right, using up nearly 100% of the SCN^– ions. Therefore, for every one mole of SCN^– reacted, one mole of FeNCS²⁺ is produced and [FeNCS²⁺]std can be assumed equal to [SCN^–]_i, the initial concentration of SCN⁻ in the standard solution.