smaerd
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Spectrophotometry Lab Need Help!
So we used a spec. 20 in my last lab for class which I must say was bad ass! Although I'm pretty damn confused here interperating the data. I did a
bunch of research but I'm still stuck.
My professor made no mention of the cuvette's diameter nor radius so finding B for Beer's Law is pretty impossible. I want to assume they were 1cm
inner diameter?
Anyways it says to plot Absorbance verses [FeNCS2+]. Which is all fine and dandy for absorbance, we have those values recorded along with the
Transmittance %. Which is kind of redundant because Absorbance ~ -log(T). We also weren't given the molar absorptivity? (just trying to show you guys
I've been researching this I'm just stuck).
The equilibrium equation is: Keq = [FeNCS2+] / [SCN-] * [Fe3+]
The initial [SCN-] is known, so is the [Fe(NO3)3] and obviously the initial [FeNCS2+] = 0M. The volumes of the solutions are known to calculate
concentrations. Finding the concentration of FeNCS2+ is not known and I'm a little stuck on how to do this.
Any help is greatly appreciated, my lab group would be very appreciative!
[Edited on 6-3-2011 by smaerd]
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smaerd
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Is the percent transmittance the change/delta in molar concentration for the [FeNCS2+]?
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tmb
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Hi. I'm pretty sure that what you need to do is:
Plot a calibration curve (line) for absorbance vs. [FeNCS2+], using the absorbances from the known concentrations of [FeNCS2+].
Using the calibration graph, determine the concentration of your unknown by reading across on the graph - using your (experimentally-determined)
absorbance value.
Use this value as [FeNCS2+] in the equation - and the known (initial) [SCN-] and [Fe(NO3)3] for the other values.
Beer-Lambert law is very seldom used in this kind of experimental calculation since, as you observed, the length of the cell is invariably 1cm to
allow for easy calculations.
Good luck.
[Edited on 6-3-2011 by tmb]
[Edited on 6-3-2011 by tmb]
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smaerd
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The problem I'm having is nowhere in the lab-book is there given known [FeNCS2+] values? I know it's a linear relationship. I just tried deriving an
ICE table(which didn't work obviously), that's when you know your thinking too hard about something...
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smaerd
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Couldn't I use a trial/flask that had 4*10^(-5)M concentration of SCN- and a 0.04M concentration of Fe(NO3)3 to determine that the equilibrium has
been shifted all the way to the right which would give nearly 4*10-5M concentration of FeSCN2+? This would be 1000 times the excess of Fe3+ ions.
So if the concentration from that is had, then I could calculate the Molar Absorptivity using Beer's Law and then use this value to calculate the
missing Concentrations?
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smaerd
International Hazard
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Registered: 23-1-2010
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Mood: hmm...
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Oh wow nvm I am retarded...
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