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Offline _cheers

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finding mass
« on: January 21, 2007, 10:36:02 PM »
Any clues how to find the mass of an aspirin in a tablet dissolved in solution to .03598 mg/ml? The mass of the table was 359.8 mg and the actual mass of aspirin/tablet is (as on the bottle) 325 mg. Absorbance was .301

hints? suggestions on how to proceed?
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Offline Mitch

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Re: finding mass
« Reply #1 on: January 21, 2007, 11:32:26 PM »
The mass isn't on the bottle?
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Offline _cheers

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Re: finding mass
« Reply #2 on: January 21, 2007, 11:37:09 PM »
I wish, but its a spectrophotometric lab, if that helps. We did the standardized solutions with an Fe3+ solution, did the Beer's Law Plot with concentrations and absorbance. I know I need to somehow use that plot and line equation to find the mass of ASA in the sample solution and use that to somehow equate the mass in the real ASA tablet. I have not a clue! We never learned any of this! *delete me*
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Offline Borek

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Re: finding mass
« Reply #3 on: January 22, 2007, 03:52:18 AM »
You can read the concentration from the calibration curve, then convert concentration to mass.
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Offline _cheers

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Re: finding mass
« Reply #4 on: January 22, 2007, 07:47:26 PM »
ok, I have my concentration at .03292 mg/ml, but I really dont know how to convert this to mass! ??? ??? ??? ??? ??? and I'm going to cry!
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Offline Dan

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Re: finding mass
« Reply #5 on: January 22, 2007, 07:49:53 PM »
Hint: what's the molecular mass of aspirin? (wikipedia)
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Offline _cheers

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Re: finding mass
« Reply #6 on: January 22, 2007, 08:03:07 PM »
180 g/mol

but what do I do with the mg/ml? Do I use the original mg of the aspirin (398.9 mg) x mole/180 g/mol..giving me moles of the original tablet, but not the moles in solution after its been diluted 2X which gave me the concentration that I have. Nope..no idea! :-[
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Offline Dan

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Re: finding mass
« Reply #7 on: January 22, 2007, 08:18:20 PM »
sorry, I was skimming, I missed that you conc was mg/mL. You need to know the volume (mL) to work out mass (mg)

Iff you know conc in (mg/mL) and volume in (mL), how can you find mass (mg)?
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Offline _cheers

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Re: finding mass
« Reply #8 on: January 22, 2007, 08:28:15 PM »
Ok, it was originally diluted in 250 ml of water, but then we took out 25, diluted to 100, then took out 10 and diluted again to 100. Which volume measure do I use? and then do I just X the concentration by the ml to get mg?
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Offline Dan

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Re: finding mass
« Reply #9 on: January 23, 2007, 04:29:20 AM »
You have the conc of the final diluted solution (the one you analysed). You have to work back through the dilutions, and work out the conc of your original 250mL solution, and then get the mass.
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