To determine what permits the assumption, we need to analyze the given information. The standard solution is prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 with 1.00 mL of 0.0020 M KSCN.
Considering the given concentrations, let's first calculate the moles of Fe(NO3)3 and KSCN used:
Moles of Fe(NO3)3 = (concentration of Fe(NO3)3) x (volume of Fe(NO3)3)
Moles of Fe(NO3)3 = (0.200 M) x (0.00900 L) = 0.0018 mol
Moles of KSCN = (concentration of KSCN) x (volume of KSCN)
Moles of KSCN = (0.0020 M) x (0.00100 L) = 0.0000020 mol
Now, let's consider the balanced equation for the reaction:
Fe3+ + SCN- ↔ FeSCN2+
From the balanced equation, we can see that the molar ratio between Fe(NO3)3 and KSCN is 1:1. Therefore, the limiting reagent in this reaction is KSCN as it is present in a much smaller quantity.
Since the limiting reagent is KSCN, it will be completely consumed in the reaction, leaving no excess KSCN in the solution. Consequently, all the moles of Fe(NO3)3 will react with the moles of KSCN present, leading to the formation of FeSCN2+.
Based on this, the assumption made in this standard solution is that all the KSCN reacts to form FeSCN2+, and there is no excess Fe(NO3)3 or unreacted KSCN left in the solution. This assumption allows us to determine the equilibrium concentration of FeSCN2+ ([FeSCN2+]std) using the stoichiometry of the reaction.
Therefore, the option that permits this assumption is D. The excess Fe3+ prevents the formation of the neutral Fe(SCN)3.