Question

Use the mean value that you determined for Keq to calculate the SCN- concentration in a solution whose initial Fe3+ concentration was 4.00x10^-2 M (.04M) and initial SCN- concentration was 1.00x10^-3M (.001M). Is all the SCN- in the form of FeNCS2+? Does the result of the calculation in question 3 justify your original assumption that all of the SCN- is in the form of FENSS+2? My Keq is 378. I would greatly appreciate it if you showed every step. Thank You in advance!

Answer 1

the use of an ICE chart would help here

Fe 3+ + SCN- --> FeSCN 2+

.......Fe 3+ .......SCN-.........FeSCN 2+
I.......0.04..........0.001..............
C........-x...............-x.............
E.....0.04-x.....0.001-x...........x

Keq = 378.1= x / (0.04-x)(0.001-x)
378.1 = x / (x^2 - 0.041x + 4x10^-5)
378.1x^2 - 15.5021x + 0.015124 = x
378.1x^2 - 15.5021x + 0.015124 = 0
x = -0.001M or 0.04M
so, using your Keq, there would be no SCN- or Fe 3+ left.....all would be in the form of FeSCN 2+