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### Topic: calculating sample weight  (Read 10586 times)

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#### dnbwise ##### calculating sample weight
« on: December 01, 2005, 08:36:38 PM »
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?

#### mike ##### Re:calculating sample weight
« Reply #1 on: December 01, 2005, 09:02:55 PM »
Why can't you just weigh the sample?
There is no science without fancy, and no art without facts.

#### dnbwise ##### Re:calculating sample weight
« Reply #2 on: December 01, 2005, 09:18:21 PM »
I can't weight the sample because the experiment has already taken place for one.

#### mike ##### Re:calculating sample weight
« Reply #3 on: December 01, 2005, 09:31:23 PM »
You could still figure out K per serving but using volume instead of mass maybe.
There is no science without fancy, and no art without facts.

#### dnbwise ##### Re:calculating sample weight
« Reply #4 on: December 01, 2005, 11:14:16 PM »
how would I go about doing so?

#### mike ##### Re:calculating sample weight
« Reply #5 on: December 01, 2005, 11:20:31 PM »
According to your results, how much K was there in the sample that you measured?
There is no science without fancy, and no art without facts.

#### dnbwise ##### Re:calculating sample weight
« Reply #6 on: December 01, 2005, 11:36:24 PM »
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.

#### mike ##### Re:calculating sample weight
« Reply #7 on: December 01, 2005, 11:51:37 PM »
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.
There is no science without fancy, and no art without facts.

#### dnbwise ##### Re:calculating sample weight
« Reply #8 on: December 02, 2005, 01:12:30 AM »
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

« Last Edit: December 02, 2005, 01:19:51 AM by dnbwise »

#### mike ##### Re:calculating sample weight
« Reply #9 on: December 02, 2005, 01:19:21 AM »
If I have read your data correctly, sample A showed an absorbance of 0.364.

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.
There is no science without fancy, and no art without facts.

#### dnbwise ##### Re:calculating sample weight
« Reply #10 on: December 02, 2005, 01:38:13 AM »
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)?

#### dnbwise ##### Re:calculating sample weight
« Reply #11 on: December 02, 2005, 10:00:31 PM »
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.

#### mike ##### Re:calculating sample weight
« Reply #12 on: December 04, 2005, 07:37:19 PM »
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  There is no science without fancy, and no art without facts.

#### dnbwise ##### Re:calculating sample weight
« Reply #13 on: December 06, 2005, 02:24:38 AM »
all good man, I figured it out the other day, but thanks.