Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: dnbwise on December 01, 2005, 08:36:38 PM
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I did a spectrometry experiment measuring the absorption of K+ samples of gatorade. I added 4ml of gatorade per sample to a 100 ml volumetric flask, and filled to the mark with DI water. Once I got the absorption I am supposed to figure out how much K per serving but I need to know the sample weight so I can calculate the K per serving. How can I figure out the sample weight, it should be around 4 ml, since that is what I added with a pipet, but I need to know the exact amount?
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Why can't you just weigh the sample?
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I can't weight the sample because the experiment has already taken place for one.
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You could still figure out K per serving but using volume instead of mass maybe.
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how would I go about doing so?
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According to your results, how much K was there in the sample that you measured?
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I have the absorbance for the samples
sample 1: A = 0.364
sample 2: A = 0.427
from these values I can use the equation from my calibration curve (A = .0452*[K] + .086) to determine the [K] in ppm for the samples.
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Great, so you know the concentration in your diluted sample (remember 4mL in 100mL) so you just need to calculate the concentration in the original undiluted 4mL of gatorade.
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wouldn't that just give me the K weight in the sample? The only way I know how to determine the K/serving is:
X/g serving = g of K/ g of sample => X = g of K/serving
Since, I don't know the sample weight I have two unknowns (X and g of sample). The only thing I can figure is I could say density of gatorade ~1g/L and since I used a pipet which is an accurate instrument I could figure the original sample weight by
4*10^-3 L gatorade (1 g/ L) = 4*10^-3 g gatorade
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If I have read your data correctly, sample A showed an absorbance of 0.364.
And according to your equation:
A = .0452*[K] + .086
therefore:
[K] = (A-0.086)/0.0452
to give you [K] in ppm
now ppm = mg/L
so you can calculate the [K] in mg/L for the cilute sample, you diluted 4mL to 100mL so it is really 25 times more concentrated than this. So you can calculate the [K] in the original sample of 4mL and then depending on what your sample size is (for example 300mL) you can say how much K is in the sample.
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Ok so here is what I calculate for sample 1:
[K] = .364 - .086/.0452 = 6.15 ppm
4ml (6.15 ug/ml) = .0256 mg K
1 serving of gatorade = 240 ml
How can I find the K mg/serving of this sample without the total sample weight (not just the K weight in the sample)?
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Alright, I have to figure this out by sunday, so I can write my lab report (I have way too much preparing to do for my finals this weekend than to worry about this final lab report for a 1 credit hour class). Maybe there is some easier way to calculate the K/serving with the data that I have by just using it in mL, but I only know how to do it using grams. So, basically if someone knows how to figure it out I would greatly appreciate it if they could show an example calculation. Otherwise, what do you guys think the density of gatorade is? I figure it to be the same as water, since that is what most of it is. Or maybe one of you guys could actually test this if you have a scale and a pipet.
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I don't see the problem, you know the concentration of K in mg/L, you know how many litres is equal to one serve ??? ???
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all good man, I figured it out the other day, but thanks.