[gianator]

Hello,

I need help with this lab report on calculating Kc, when Fe(III) and thiocyanate is involved.

I know how to calculate K_c when the coefficients of the reactants are in the 1to1 ratio, but our professor wants us to calculate it when the coefficient of SCN^- is 2 and 3 and I have no idea how to do that.

This is the data I have

Absorbance is 0.181
Fe³⁺ = 2.86 x 10^-4 M
SCN^- = 1.71 x 10^-3 M
I calculated the K_c to be 68.6

Can someone help me?
Thanks

[Borek]

Write reaction, write reaction quotient.

[gianator]

The reaction is Fe³⁺ + 2SCN^- --> FeSCN¹⁺

The reaction quotient is FeSCN¹⁺/Fe³⁺ * (SCN^-)²

That part I get, but what do I plug in into that reaction quotient?

For the one to one ratio, I had divide the absorbance 0.183 by 6120 to get 2.96 x 10^-5, then I subtracted that from 2.86 x 10^-4 to get 2.56 x 10^-4 and I also subtracted that from 1.71 x 1^-3 to get 1.68 x 10^-3

Then I set up the reaction quotient

(2.96 x 10^-5) / (2.56 x 10^-4) * (1.68 x 10^-3) = 68.6

Do I just square (1.68 x 10^-3) to get the answer when the coefficent is 2?

[Borek]

FeSCN¹⁺

Rather Fe(SCN)₂⁺

The reaction quotient is FeSCN¹⁺/Fe³⁺ * (SCN^-)²

OK (apart from the compound name, as signalled above).

but what do I plug in into that reaction quotient?

Equilibrium concentrations. You should know them from your experimental data. Note, that your calculations as posted don't make much sense. They can be right, but we have no idea what you did, so we can't know what and why you are subtracting/multiplying and so on.