[nozo]

Hi, I'm trying to solve for Kc.. can someone please check my work? I think I am doing something horribly wrong... 

Fe⁺³ + SCN^- <----> FeSCN⁺²

Vol Fe(NO₃)³ = 5 ml; C = .002 M
Vol KSCN = 1.15 ml; C = .002 M
Vol H₂O = 3.85 ml; C = .002 M
C [FeSCN⁺²] = .0000388 M (derived from caculating absorbance * .0002 M/A)

Here's my work:

total # of ml in soln = 10 ml

initial # of mols:
Fe⁺³ = (.002 M * .0005 L) = .000001
SCN^- = (.002 M * .00115 L) = .0000023
FeSCN = (.002 M * 6.15 ml = C * 10 ml) = .00123

# of mols at equil:
FeSCN = .0000388 * .01 L = .000000388
Fe⁺³ = (.000000388 - .000001) = .00000012
SCN^- = (.000000388 - .0000023) = .000001912

equilibrium concentrations:
Fe⁺³ = .00000012 / .01 L = .000000012
SCN^- = .000001912 / .01 L = .000000191
FeSCN = .0000388

Kc = [.0000388] / [.00000012] * [.000000191] = 1.69 x 10^{^9}

I think I am waaay off... 

[sdekivit]

you have a wrong amount of mol FeSCN²⁺. Notice that at equilibrium, the volume is 10 mL

Thereby you calculate the equilibriumconcentration for Fe³⁺ and SCN^- incorrectly:

for example Fe³⁺ should be 0,000001 - 0,000000388 mol and not the way you did it

And (it's not wrong what you did though) because al the mols are in a same volume you can calculate K_c with the mols since all numbers are divided by the same number. Please place brackets in your calculation for K_c !

[nozo]

Hi, I don't see how my concentration for FeSCN²⁺ is wrong.... I solved it using the given eqn:

[FeSCN²⁺] = (absorbance) * (2x10^-4 M/A)

My absorbance was .194 from taking the log of my T-samples which was 100/64

Or do you mean to say that my initial mols for FeSCN is wrong?

Ack.. I am so confused..   

[sdekivit]

sorry i first had another reply and i modified it. It should be mols FeSCN²⁺