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 0.510. If a trial's absorbance is measured to be 0.250 and its initial concentration of SCN¡V was 0.00050 M, the equilibrium concentration of SCN¡V will be ?
Use Beer's Law and the standard to determine the constant a in A = abc. You know A, you know b, the cell length, and you can calculate c. Then turn your attention to the unknown and use A = abc.
You know A from the problem, a from the previous calculation, b the cell length, and solve for c, the concn of the unknown.If you know the initial concn of SCN^- and you know how much was used to form the FeSCN+2 complex, you can calculate the concn of SCN^- remaining at equilibrium.
To find the equilibrium concentration of SCN- in this solution, we can use the concept of Beer-Lambert Law.
According to the Beer-Lambert Law, the absorbance of a solution is directly proportional to the concentration of the absorbing species and the path length of the sample. The equation is given by:
A = εlc
Where:
A is the absorbance,
ε is the molar absorptivity (a constant specific to the absorbing species and the wavelength of light used),
l is the path length (in this case, it is not provided but is assumed to be 1 cm),
c is the concentration of the absorbing species.
Now, in this problem, we are given the absorbance of the solution and the initial concentration of SCN-. We can rearrange the Beer-Lambert Law equation to solve for concentration:
c = A / (εl)
The molar absorptivity (ε) for FeSCN2+ is a constant that can be determined experimentally. In this case, we are not given the value of ε, so we cannot calculate the exact concentration of SCN-.
However, we can determine the relationship between the concentrations and absorbances using the given standard solution. The standard solution has an absorbance of 0.510 and a known concentration of SCN-. By plugging in these values into the rearranged equation, we can solve for ε:
ε = A / (cl)
Since we know A = 0.510, c = 0.00050 M (given), and l = 1 cm, we can calculate ε:
ε = 0.510 / (0.00050 x 1)
Once we have the value of ε, we can use the same equation to calculate the equilibrium concentration of SCN- given the absorbance of the trial solution. In this case, the absorbance of the trial solution is 0.250:
c = A / (εl)
c = 0.250 / (ε x 1)
Now, substitute the value of ε that we calculated earlier into the equation, and solve for c. This will give us the equilibrium concentration of SCN-.