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 think i have to use mv=mv, but i don't know how to relate absorbance on this problem.
The Beer-Lambert Law.
A = abc.
Use A = abc for the standard to calculate the constant, a (the absorptivity). b = cell length (but you don't need to use that IF both standard and unknown were measured using the same cell. Then use A= abc on the unknown to calculate FeSCN^+2 in the final solution. Knowing how much that is will let you determine the amount of KSCN remaining unreacted in the final solution. Notice that you obtain c (the concn of the standard by mols/L = M. Note also that the standard solution is prepared and you must calcualte molarity; i.e., it isn't 0.200 M. Post your work if you get stuck.
To solve this problem, you need to apply the Beer-Lambert Law and the concept of molar ratios. The initial absorbance (A₀) and the initial concentration of SCN– (C₀) are given, and you are asked to find the equilibrium concentration of SCN–.
The Beer-Lambert Law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species and the path length of the sample. Mathematically, it can be written as:
A = εlc
Where:
A is the absorbance,
ε is the molar absorptivity (a constant),
l is the path length (usually given in cm),
c is the concentration.
In this problem, the absorbance value (A) of the standard solution is given as 0.520. First, you need to calculate the concentration of FeSCN²⁺ in the standard solution using the Beer-Lambert Law. Since the path length is not given, you can assume it to be 1 cm for simplicity.
A = εlc
0.520 = ε × 1 × c
Solving for c, we get:
c = 0.520/ε₁
Next, you need to calculate the concentration of FeSCN²⁺ in the measured solution using the absorbance value of 0.275. Again, assuming the path length to be 1 cm:
A = εlc
0.275 = ε × 1 × c
Solving for c, we get:
c = 0.275/ε₂
Now, we can set up an equation using the molar ratios of SCN⁻ between the standard solution and the measured solution.
Since the standard solution is prepared by combining 9.00 mL of 0.200 M Fe(NO₃)₃ and 1.00 mL of 0.0020 M KSCN, we can calculate the molar ratio of SCN⁻ between FeSCN²⁺ and KSCN.
Molar ratio of SCN⁻ = (moles of SCN⁻ in FeSCN²⁺) / (moles of SCN⁻ in KSCN)
To calculate the moles of SCN⁻ in FeSCN²⁺ in the standard solution, you can use the molar ratio stoichiometry from the balanced equation of the reaction between Fe³⁺ and SCN⁻ to form FeSCN²⁺.
Once you have the moles of SCN⁻ in FeSCN²⁺, you can use it to calculate the moles of SCN⁻ in the measured solution by multiplying it by the molar ratio.
Now, you can use the moles of SCN⁻ in the measured solution to calculate the equilibrium concentration of SCN⁻.
Equilibrium concentration of SCN⁻ = (moles of SCN⁻ in measured solution) / total volume of measured solution
Substituting the values into the equation will give you the equilibrium concentration of SCN⁻.