Question

Calculate the initial molarity for Fe3+ and SCN- in each of the 6 vials. Using the molarity of FeSCN2+ at equilibrium in each of the six trials, determine the molarity of each reactant when equilibrium is reached.

I'm really confused on how to do this. Could someone please give me an example of just how to do the first one?

Data I have:

- initial concentration of Fe3+ = 1.8 * 10^-4 M

- initial concentration of SCN- = 2.0 * 10^-7 M

- initial concentration of FeSCN2+ = 2.0 * 10^-7 M.

- molar absoptivity = 7.7 * 10^-3 L/mol*cm

- the first vial contains 1 mL of Fe(NO3) and 9 mL of KSCN

- for the first vial, l = 470 nm, and A = 0.0735

Please do your best to explain it thoroughly so I can repeat the process for the other five vials. Thank you very much.

Answer 1

Although, there is a confusion about the question.Initial Molarity vs Equlibrium Molarity of Fe(SCN)2+, I showed the following calculation from the first line of your question.

1 ml 18 x 10^-4 M Fe(NO3)2 = (1x 18 x 10^-4)/1000 moles Fe(NO3)2.

[From the definition of molarity]

9 ml 2.0 x 10^-7M KSCN = (9x 2.0 x 10^-7)/1000 moles KSCN.

Total volume of the solution = 1+9 =10 ml.

10 ml solution contains (1x 18 x 10^-4)/1000 moles Fe(NO3)2

1000 ml solution contains (1x 18 x 10^-4)x1000/10x1000 moles Fe(NO3)2=1.8 x 10^-4 M.

10 ml solution contains (9x 2.0 x 10^-7)/1000 moles KSCN.

1000 ml solution contains (9x 2.0 x 10^-7)x1000/10x1000 moles KSCN=1.8x10^-7M.

Initial Molarity of the two solutions :1.8 x 10^-4 M and 1.8x10^-7M