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 0.500. If a trial's absorbance is measured to be 0.310 and its initial concentration of SCN– was 0.0010 M, the equilibrium concentration of SCN– will be
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To find the equilibrium concentration of SCN–, we can use the concept of Beer-Lambert Law. The Beer-Lambert Law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species.
In this case, the standard solution has an absorbance of 0.500, which we can use as a reference. The trial solution has an absorbance of 0.310, which we can use to calculate its concentration.
To calculate the concentration of SCN– in the trial solution, we can rearrange the Beer-Lambert Law equation as follows:
A = εbc
Where:
A = absorbance of the solution
ε = molar absorptivity constant (a constant specific to the substance being measured)
b = path length of light through the solution (in cm)
c = concentration of the absorbing species (in mol/L)
We know the absorbance (A) of the trial solution is 0.310, and the initial concentration (c) of SCN– is 0.0010 M. However, we need to determine the path length (b) for this particular measurement.
The path length may vary depending on the cuvette used or the instrument's specifications. A common path length for a cuvette is 1 cm. If your experiment used a different path length, substitute that value.
Now, we can substitute the values into the equation and solve for the path length (b):
0.310 = ε * b * 0.0010
Solving for b:
b = 0.310 / (ε * 0.0010)
Now that we have determined the path length, we can use it to find the equilibrium concentration of SCN– in the trial solution.
In the standard solution, we combined 9.00 mL of 0.200 M Fe(NO3)3 with 1.00 mL of 0.0020 M KSCN. The final volume of the combined solutions is 10.00 mL.
To find the equilibrium concentration of SCN–, we can write an equation based on the balanced chemical equation of the reaction:
Fe3+ + SCN– → Fe(SCN)2+
Since the mole ratio between Fe3+ and SCN– is 1:1, the equilibrium concentration of SCN– in the standard solution is the same as the initial concentration of KSCN used, which is 0.0020 M.
Now we have all the information needed to calculate the equilibrium concentration of SCN– in the trial solution:
Equilibrium concentration of SCN– = (0.310 / (ε * b)) * (0.0020 / 10.00)
Substitute the value of b, the molar absorptivity constant (ε) for SCN–, and solve the equation to get the equilibrium concentration of SCN– in the trial solution.