Question

Operating Wavelength : 458.7 nm

Preparation of Standard Calibration Curve of [FeSCN]²⁺

Sample Number	Absorbance	Final Concentration of [FeSCN]2+(M)
A	0.157	4.0E-5
B	0.272	8.0E-5
C	0.422	1.2E-4
D	0.615	1.6E-4
E	0.722	2.0E-4

Determination of [FeSCN]²⁺ in Equilibrium Mixtures

Sample Number	Absorbance
1	0.240
2	0.334
3	0.436
4	0.544

Determination of [FeSCN]²⁺ in equilibrium mixtures

For Sample #1: Enter the initial concentration of Fe3+

Calculate the equilibrium concentration of Fe3+

Enter the initial concentration of HSCN

Calculate the equilibrium concentration of HSCN

Calculate Keq for sample 1

Answer 1

The Absorbance (A) is related to concentration (c) as follows

where is the absorptivity and l is the length of the cell

Now plot Absorbance vs concentration in a graph.

Fit a line with intercept 0 and find the equation. Note its slope. the slope of the line will give us the value of which we can use in further calculation. This grap with the equation is shown below

Therefore .

For sample 1 A = 0.240,

Therefore concentration of Fe[SCN]²⁺ can be found as follows

The values for inital concentration of Fe³⁺ and HSCN are not given.

Let initial concentration of Fe³⁺ is a and that of HSCN is b.

Then final concentration of Fe³⁺ = a-6.57x10-5

final concentration of HSCN = b-6.57x10-5

The equillibrium constant K_eq can be found as follows