Sven
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Question relating to absorbance of UV radiation
So i've been working on an assignment for a few days now, and have hit a large hurdle.
"At pH 13, the absorbance of a particular phenolsolution is 1.5 at 400 nm and 0.0 at 270 nm. At pH 4, the values for the solution of the same
cocentrations are 0.0 and 1.0 at these two wavelengths respectively. At pH 9, the values are 0.9 and 0.4 respectively.
(a) Explain these spectral changes
(b) Calculate the pKa of the phenol.
(c) If the concentration used was 18.8 mg of phenol in 20 ml of solvent, a cell length of 1cm was used and ε270 = 100L.mol-1.cm-1 calculate the
molecular weight of the phenol."
So the first two parts were fine, but then it gets to the third question, which at first inspection looks quite easy, but isnt. Well it probably is,
but I can't seem to see it.
Also, I don't want the actual answer. Just a step in the right direction 
Thanks guys, nice forum too
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Proteios
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beers law takes you from abs. to conc. in mol/litre
you have conc. in g/litre
mol. mass is just one divided by the other
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DDTea
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I suppose this would be more fit for the "Beginning's" section. However, I have a useful tip for you in future chemistry problems...
When in doubt, find the number of moles
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chemoleo
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Wow I thought the last part was easiest...
But yes, with the data you have, there is all the info you need. the extinction coefficient relates to moles, and the mg and Avogadros number should
be sufficient to get the MW.
However, as you seem to know the answer to question 1, could you write it down? You obviously get a blueshift, presumably because things get
protonated at low pH, while at high pH it is ionised - but why does this cause a blueshift/difference in absorbtive spectra? The physical meaning
behind it?
Yes, and this should have gone in the homeworks section!
[Edited on 7-5-2004 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Sven
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Hah, how on earth did I not see that?
The mind boggles.
Thanks guys
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chemoleo
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so what about the answer to question 1 ??
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Sven
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| Quote: | Originally posted by chemoleo
so what about the answer to question 1 ?? |
Sorry chemleo, I was in a hurry when I posted before.
Basically, the newly formed pair of non-bonding electrons on the oxygen atom (following deprotonation) provides extra stability to the electronically
excited state, thus a hypsochromic (blue) shift occurs. (ie less energy is required to induce an electronic excitation, thus the wavelength is higher)
(Or so I think  Someone may be able to give a better explanation)
[Edited on 8-5-2004 by Sven]
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