Fe3+ is added to excess Ag+ prior to titration with KSCN. When all the excess Ag+ has reacted, the red complex of Fe(SCN)2+ forms. This red complex is visible when the [SCN-] is about 2x10^-4M. at this pt. >99% (but not all) of the Ag+ has precipitated. Calc. the Ag+ when the Fe(SCN)2+ forms.
To calculate the concentration of Ag+ when the Fe(SCN)2+ complex forms, we need to use the information given in the problem statement.
The problem states that when the red complex Fe(SCN)2+ forms, the concentration of SCN- ([SCN-]) is about 2x10^-4M, and at this point, >99% of Ag+ has precipitated.
Let's break down the problem and understand the steps to solve it:
1. First, we need to calculate the concentration of Ag+ remaining in the solution when the Fe(SCN)2+ complex forms.
2. We can assume that the initial concentration of Ag+ is much larger than the concentration of SCN-. Therefore, we can assume that most of the Ag+ reacts to form the AgSCN precipitate and that only a small portion remains in the solution.
3. Since >99% of Ag+ has precipitated when the Fe(SCN)2+ complex forms, we can calculate the remaining Ag+ concentration by subtracting the precipitated amount from the initial Ag+ concentration.
4. To do this, we need to find the concentration of SCN- required to precipitate >99% of Ag+ and use it to calculate the concentration of Ag+.
Now, let's proceed with the calculations:
Step 1: Calculate the concentration of Ag+ that has precipitated.
Since >99% of Ag+ has precipitated, the remaining Ag+ concentration is less than 1% of the initial concentration. Therefore, we can assume that the concentration of Ag+ is negligible compared to the initial concentration. Thus, the remaining Ag+ concentration can be approximated as zero.
Step 2: Calculate the concentration of SCN- required to precipitate >99% of Ag+.
To precipitate >99% of Ag+, we need to determine the minimum concentration of SCN- that will react with the Ag+ to form AgSCN. Since the molar ratio between Ag+ and SCN- is 1:1, the concentration of SCN- required will be equal to the concentration of Ag+ that precipitates.
Step 3: Use the SCN- concentration to calculate the Ag+ when Fe(SCN)2+ complex forms.
From the problem statement, the concentration of SCN- ([SCN-]) is given to be about 2x10^-4M when the Fe(SCN)2+ complex forms. This concentration represents the concentration of Ag+ that will remain in solution.
Therefore, when the Fe(SCN)2+ complex forms, the concentration of Ag+ will be approximately equal to the concentration of SCN-, which is 2x10^-4M.
Hence, the Ag+ concentration when the Fe(SCN)2+ forms is approximately 2x10^-4M.