I thought I was close to figuring out the answer but when I try to follow along

Question

I thought I was close to figuring out the answer but when I try to follow along from my notes, I'm way off. Can someone walk me through this?

A student created standards in order to make a calibration curve. The following volume of 0.0020 M SCN was diluted with 0.20 M Fc3+ to make 25.0 mL standard solutions. Assume all the SCN is complexed as FescN once the standards are made. Standard A Standard B Standard C Standard D Standard E Standard F Approximate [Fc3+l in 0.20 0.20 0.20 0.20 0.0020 M SCN added Mol SCN 0.27 mL 0.54 mL 0.81 ml 1.08 mL 1.35 mL 2.70 mL Previeww Previeww [SCN1 ion in solution [FescN in Previeww Preview Previcww Previcww Previcww Previcw

Explanation / Answer

The no. of moles of SCN- added:

Standard A: 0.002 mol/L*0.27*10-3 L = 5.4*10-7 mol

Standard B: 0.002 mol/L*0.54*10-3 L = 1.08*10-6 mol

Standard C: 0.002 mol/L*0.81*10-3 L = 1.62*10-6 mol

Standard D: 0.002 mol/L*1.08*10-3 L = 2.16*10-6 mol

Standard E: 0.002 mol/L*1.35*10-3 L = 2.7*10-6 mol

Standard F: 0.002 mol/L*2.7*10-3 L = 5.4*10-6 mol

[SCN-] in 25 mL standard solutions:

Standard A: (5.4*10-7 mol/25 L)*1000 = 2.16*10-5 M

Standard B: (1.08*10-6 mol/25 L)*1000 = 4.32*10-5 M

Standard C: (1.62*10-6 mol/25 L)*1000 = 6.48*10-5 M

Standard D: (2.16*10-6 mol/25 L)*1000 = 8.64*10-5 M

Standard E: (2.7*10-6 mol/25 L)*1000 = 2.16*10-4 M

Standard F: (5.4*10-6 mol/25 L)*1000 = 2.16*10-4 M

[FeSCN2+] in 25 mL standard solutions:

Since SCN- is the limiting reagent as it's consumed completely in the titration, the [FeSCN2+] = [SCN-]

Reason: The no. of moles of SCN- reacts with the same no. of moles of Fe3+ to form the same no. of moles of FeSCN2+ i.e. nFeSCN2+ = nSCN-

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