[purpTerp]

I've been pounding my head against the wall the past couple of days due to having an issue with finding the equilibrium value from a concentration versus absorbance lab.

In the lab here are the readings I acquired:

TT#	[Fe+3] init	[SCN-] init	[FeSCN+2] x	[FeSCN+2]	Absorbance
1	0	0.50M	0	N/A	0
2	2.0*10-5M	0.50M	2.0*10-5M	N/A	0.17
3	4.0*10-5M	0.50M	4.0*10-5M	N/A	0.36
4	6.0*10-5M	0.50M	6.0*10-5M	N/A	0.55
5	8.0*10-5M	0.50M	8.0*10-5M	N/A	0.72
6	3.6*10-4M	3.6*10-4M	N/A	1.76M (calculated in section below)	0.135
7	3.6*10-4M	7.1*10-4M	N/A	2.72M (calculated in section below)	0.310
8	6.3*10-4M	6.3*10-4M	N/A	3.14M (calculated in section below)	0.385
9	6.3*10-4M	9.4*10-4M	N/A	4.43M (calculated in section below)	0.62

Calculation of of slope from excel graph (concentration versus absorbance):
Slope = 0.182

Calculation of concentration of [FeSCN⁺²] at Equilibrium
6)  C = (0.135 -(-0.186)) / 0.182 = 1.76M FeSCN
7)  C = (0.310 -(-0.186)) / 0.182 = 2.72M FeSCN
  C = (0.385 -(-0.186)) / 0.182 = 3.14M FeSCN
9)  C = (0.62 -(-0.186)) / 0.182 = 4.43M FeSCN

When I put the value of the equilibrium concentration into the ice table and solve for x, x is coming out as a postive value for FeSCN and a negative value when I solve for Fe and for SCN...help me Obiwan, you're my only hope!

[mjc123]

What on earth are you doing? Where did you get the numbers 0.182 and -0.186? It must be obvious that if you get an absorbance of 0.72 for a concentration of 8*10^-5M, the slope is orders of magnitude different from 0.182. And how can you possibly get 1.76M complex when you started with 3.6*10^-4M of metal and ligand, and the absorption is comparable to what you got with 2*10^-5M?
Please explain exactly how you got those numbers - what you plotted against what, and what calculations you did. Then perhaps we can start to help you.