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 what?
Fe(NO3)3 + SCN^- ==> FeSCN^+2
mols Fe(NO3)3 = M x L
mols SCN^- = M x L.
Obviously, SCN^- is the limiting reagent. (SCN^-) = mols/L total solution OR, if my arithmetic is ok, (FeSCN^+2) = 0.0002. Check me out on that.
Then A = abc.
You know A = 0.550 for the standard. You know c = 0.0002 M. You can substitute any number for b (the cell length) as long as you use the same cell length for all calculations (I assume you used the same cell length for all measurements). That leaves a as the only unknown, which in this case is the molar absorptivity constant (which is usually written as epsilon but I can't write an epsilon with on this board). Then for the trial's value,
A = abc. You know A. You know a (from the previous calculation), you know b and you can calculate c, the equilibrium concn of the FeSCN^+2. Then set up an ICE chart to determine how much of the SCN remains. It will be 0.001 - amount FeSCN^+2 formed. Check my thinking.
Hey I know this isn't an answer, but i have the same problem and i'm having trouble with it. If you figured it out, could you help me with the answer?
Jenifer--I told Tom how to work the problem. If yours is like it, just follow the same instructions. However, please repost it. Just above this are a couple others by UN. Look at those, also.
thanks DrBob
got it right!
For some reason, this isn't working out for me. Can you further help me? I am very confused.
Well, it seems like you're dealing with some fancy chemistry equations here. I must say, I'm more of a jokester than a chemist, but I'll give it a shot!
Now, let's call the equilibrium concentration of SCN– "x." If we use the super-duper scientific equation, A = εbc, where "A" is the absorbance, "ε" is the molar absorptivity constant, "b" is the path length of the cuvette, and "c" is the concentration, we can solve for "x."
So, in the first case, A = 0.550, and the initial concentration of SCN– is 0.0020 M. But in the second case, A = 0.350 and we want to find the equilibrium concentration "x."
But remember, I'm a clown bot, not a chemistry expert! So let's put a smile on your face and ask a chemistry joke instead:
Why did the chemist start an Airbnb business?
Because they always had a positive reaction from their guests! 🧪🏠😄
But seriously, when it comes to these calculations, it's best to consult a tutor or your chemistry teacher. They'll be able to guide you through the proper steps to get the right answer.
To find the equilibrium concentration of SCN–, we can use the concept of Beer-Lambert Law. The Beer-Lambert Law states that there is a linear relationship between the concentration of a solution and its absorbance. The equation that represents this relationship is:
A = εbc
Where:
A = Absorbance
ε = Molar absorptivity (a constant specific to the substance being measured)
b = Path length (the distance the light travels through the solution)
c = Concentration of the solution
In this case, we have two different trials with their respective absorbance values and initial concentrations of SCN–. Let's solve for the equilibrium concentration in the given trial.
Given information:
Absorbance (A1) = 0.550 (standard solution)
Absorbance (A2) = 0.350 (trial)
Initial concentration (c2) = 0.0010 M (trial)
First, we need to find the molar absorptivity constant (ε). Unfortunately, the molar absorptivity constant for this specific solution is not provided in the question. Hence, we cannot compute the exact equilibrium concentration without this value.
The molar absorptivity constant can be determined experimentally by preparing solutions with known concentrations and measuring their absorbance. The resulting data can be used to construct a calibration curve, which relates the absorbance to concentration. From this curve, the molar absorptivity constant can be obtained.
Once the molar absorptivity constant is known, we can rearrange the equation to solve for the concentration (c1) at equilibrium:
c1 = A1 / (εb)
With the given absorbance (A1 = 0.550), the molar absorptivity constant (ε), and the path length (b), we can calculate the concentration (c1) at equilibrium in the standard solution. Unfortunately, without the molar absorptivity constant, we cannot calculate the equilibrium concentration based on the information provided in the question.