Question

Im still not understanding how to calculate the molarity of Fe(SCN)^2+

I have the absorbance for them but I dont know what to do from there.

I would really appreciate the help. please and thank you.

H115 lab manual_rev2016_MOD19AUG18_JSchesser.pdf (2.82 MB) Page < 65 > of 130 Tortas experiment unless you can measure their absorbance values within one hour 0 - ZOOM + Label 6 screw top test tubes A-F Using pipets, measure the solutions as outlined in the table below. Note: Two solutions of Fe(NO3), and KSCN with different concentrations are used in this experiment. Be sure you use the correct one. Solutions 1 0.00 I 5 5.00 10.00 3.00 7.00 20.00 10.00 4.00 6.00 20.00 S 10.00 2.00 8.00 20.00 10.00 1.00 9.00 20.00 10.00 0.00 10.00 20.00 .00 20.00 mL of 0.20 M Fe(NO3) 3 mL of 9.00x104 M KSCN mL of 0.50 M HNO3 Total volume (mL) Initial conc. of Fe+ (M) R itial conc/of SCN1, (M) Final conc of FeSCN. (M) Absorbance Thoroughly mix each solution by capping and inverting at least 10 times. Measure the absorbance of each solution at 510 nm. Set the spectrometer to "zero" absorbance with Solution F. Keep solution F for Part 2. Calculate the molarity of Fe(SCN)2. in each flask; assume all the SCN has reacted. 65 of 130

Answer 1

Sample A.

Millimoles of Fe³⁺ = 0.2 mmol/mL * 10 mL = 2 mmol

Millimoles of SCN^- = 9*10^-4 mmol/mL * 5 mL = 4.5*10^-3 mmol

Therefore, Millimoles of FeSCN²⁺ = same as the limiting reactant (SCN^-) = 4.5*10^-3 mmol

Hence, [FeSCN²⁺] (c) = 4.5*10^-3 mmol/20 mL = 2.25*10^-4 M

Note: M = mmol/mL or mol/L

Use the absorbance (A) and find the molar absorption coefficient ($\epsilon$) using Beer's law, i.e. A = $\epsilon$cl

Where 'l' is the pathlength = 1 cm (in general)