The standard solution of FeSCN2+ (prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 w/1.00 mL of 0.0020 M KSCN) has an absorbance of 0.550. If a trial's absorbance is measured to be 0.350 and its initial concentration of SCN– was 0.0010 M, the equilibrium concentration of SCN– will be?
Is there a formula? Or do I use beer's law?
Use Beer' law with the standard to find a in A=abc, then use the trial data to find c given A, a, and b. Then the equilibrium and ICE will tell you how much SCN^- is at equilibrium.
what is an ICE?
I - initial concentration
C - change in concentration
E - Equilibrium concentration
actually, you don't need ICE at all.
[FeNCS^2+]=A(eq)/A(std)*.0002
Plug in the absorbances and find [FeNCS^2+].
After that subtract [FeNCS^2+] from the initial concentration of SCN-.
you should get 0.000873 M
To determine the equilibrium concentration of SCN–, we can use the Beer-Lambert Law. The formula is as follows:
A = εcl
Where:
A is the absorbance of the solution,
ε is the molar absorptivity (also known as the molar absorptivity constant) for the complex
c is the concentration of the species being measured
l is the path length of the cell through which the light passes
In this case, we can rearrange the formula to solve for concentration (c):
c = A / (εl)
To solve this problem, you need to know the molar absorptivity constant (ε) for the complex FeSCN2+. The molar absorptivity constant is specific to the particular complex and the wavelength of light being used for measurement. It is typically given in units of M^-1⋅cm^-1.
Once you have the ε value, you can substitute the known values into the formula and calculate the equilibrium concentration of SCN–.