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 0.520. If a trial's absorbance is measured to be 0.275 and its initial concentration of SCN– was 0.00060 M, the equilibrium concentration of SCN– will be... ?
I am confused about the "initial" concn. I understand that it is 6 x 10^-4 M; however, nothing in the problem sets it up. The concn of SCN^- that is MEASURED (is that the same as the initial concn?) is done this way.
Concn of standard SCN^- = 0.002 M x 1/10 = 2 x 10^-4 M (STANDARD).
Sample = 2 x 10^-4 M x (0.275/0.520) = ?? M
To find the equilibrium concentration of SCN–, we can use the concept of Beer-Lambert Law and the relationship between absorbance and concentration.
The Beer-Lambert Law states that absorbance is directly proportional to the concentration of the absorbing species and the path length of the sample.
A = εlc
Where:
A = Absorbance
ε = Molar absorptivity (a constant for a given substance)
l = Path length (usually the length of the cuvette in cm)
c = Concentration of the absorbing species
In this case, we are given the absorbance of the standard solution (0.520) and the absorbance of the trial solution (0.275). We need to calculate the equilibrium concentration of SCN– using these values.
First, let's calculate the concentration of Fe3+ in the standard solution:
Given:
Volume of Fe(NO3)3 = 9.00 mL
Concentration of Fe(NO3)3 = 0.200 M
Using the equation C1V1 = C2V2, we can find the initial moles of Fe3+:
Moles of Fe(NO3)3 = (0.200 M) × (9.00 mL) = 1.80 mmol
Since Fe(NO3)3 and FeSCN2+ react in a 1:1 stoichiometric ratio, the moles of Fe3+ is also equal to the moles of FeSCN2+ in the standard solution.
Now, let's calculate the initial moles of SCN– in the trial solution:
Given:
Volume of KSCN = 1.00 mL
Concentration of KSCN = 0.0020 M
Moles of KSCN = (0.0020 M) × (1.00 mL) = 0.0020 mmol
Since KSCN and FeSCN2+ react in a 1:1 stoichiometric ratio, the moles of SCN– in the trial solution is also equal to the moles of FeSCN2+.
Now, let's calculate the concentration of SCN– in the trial solution:
Volume of Trial solution = Volume of SCN– = 1.00 mL
Concentration of SCN– = (0.0020 mmol) / (1.00 mL) = 0.0020 M
Given that the absorbance of the trial solution is 0.275, we can set up the Beer-Lambert Law equation:
A = εlc
Rearranging the equation, we have:
c = A / (εl)
Using the data given:
A = 0.275
l = path length (same as the standard solution)
ε = Molar absorptivity (same for both standard and trial solutions)
We can now calculate the equilibrium concentration of SCN–:
Equilibrium concentration of SCN– = (0.275) / (εl)
Please note that to accurately calculate the concentration, we need to know the molar absorptivity (ε) and the path length (l) for the specific setup and instrument being used.