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

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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.

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