Fish3715

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Hi-

If any of you have had the general chemistry lab where you synthesized acetylsalicylic acid from salicylic acid and had to colormetrically determine %yield and what not, do you know/remember how it's possible for the yield to be over 100%?

Here's the lab instructions from another school that does this lab, just in case viewing this refreshes your memory:

http://courses.chem.psu.edu/chem14/FormsF03/aspirinlab2_F03.pdf

I looked online at some sites and the only explanations I could find/think of are:
1) solid wasn't completely dry (found this online...not sure why this would be the answer; it would seem that having more water would give a lower percent yield)
2) part of the synthesized acetylsalicylic acid was actually salicylic acid (which has a lower molar mass).

Eh...no s
 

bewitched1081

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Originally posted by Fish3715
Hi-

If any of you have had the general chemistry lab where you synthesized acetylsalicylic acid from salicylic acid and had to colormetrically determine %yield and what not, do you know/remember how it's possible for the yield to be over 100%?

Here's the lab instructions from another school that does this lab, just in case viewing this refreshes your memory:

http://courses.chem.psu.edu/chem14/FormsF03/aspirinlab2_F03.pdf

I looked online at some sites and the only explanations I could find/think of are:
1) solid wasn't completely dry (found this online...not sure why this would be the answer; it would seem that having more water would give a lower percent yield)
2) part of the synthesizes acetylsalicylic acid was actually salicylic acid (which has a lower molar mass).

Eh...no s?.

1. water would increase the total mass, which would also translate into a higher amt of substance when converted into moles. think of it this way: is your towel heavier when it is soaked in water or when it is dry?
2. if salicylic acid has a lower molar mass than acetylsalicylic acid, then it would decrease yield because you are dividing the total product mass by the molar mass of acetylsalicylic acid. this is because the theoretical yield is calculated from the moles of limiting compound used, therefore if you used 10 moles of sali acid then you expect 10 moles of acetyl acid (if the ration is 1:1). if any starting compound is residual in the product, then it means that it was not converted into the heavier product. so when you divide the total mass by the molar mass of the heavier product it will give you a number less than 10 moles, given a 100 percent yield. ex: 10 moles sali = 10 g (it doesnt but just an ex.) 10 moles acetyl = 20 g; 7 moles acetyl + 3 moles residual sali= 17 g; 17 g product = 8.5 moles of product (acetyl); % yield = 85%
 
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Fish3715

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Originally posted by bewitched1081
1. water would increase the total mass, which would also translate into a higher amt of substance when converted into moles. think of it this way: is your towel heavier when it is soaked in water or when it is dry?

OK. I just thought that if the solid originally had more water, then the amount of actual ASA present would actually be less than what was originally thought (e.g. the balance would say .4 grams of solid, but in reality only .35 grams were actually the ASA).
Then ultimately, the end solution would be less concentrated (the absorbance measured would be higher, but the calculations would be done using the solid+water mass, so the %yield would appear lower?)

I'm probably overthinking this...
 

bewitched1081

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Originally posted by Fish3715
OK. I just thought that if the solid originally had more water, then the amount of actual ASA present would actually be less than what was originally thought (e.g. the balance would say .4 grams of solid, but in reality only .35 grams were actually the ASA).
Then ultimately, the end solution would be less concentrated (the absorbance measured would be higher, but the calculations would be done using the solid+water mass, so the %yield would appear lower?)

I'm probably overthinking this...

you wold not divide by solid + water mass because you are not supposed to have water. you only divide by the solid mass because that is what your product should be.
 

UCLAstudent

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Water was the problem for me when I did that experiment.
 

Fish3715

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Originally posted by bewitched1081
you wold not divide by solid + water mass because you are not supposed to have water. you only divide by the solid mass because that is what your product should be.

But you wouldn't know that it's still wet, right? You would divide your calculated mass (the mass you got after doing all the conversions below...I only copied and pasted them because i'm typing this up now) by the mass you weighed thinking the weighed mass was completely solid when it was actually part water...if that makes sense.

I'm pretty sure you're right; I'm just trying to understand this a little better.


Calculated mass of acetylsalicylic acid =
-Concentration of complex in cuvette (from equation of slope of the Beer?s Law plot- concentration = (absorbance -.0004) / 1520.4) ?
-Concentration of complex in the 25ml flask (same as the [] in the cuvette- the complex in the cuvette came from this flask) ?
-Moles of complex in the 25 ml flask (conc. * .025L= moles) ?
-Moles of salicylate ion in the 1 ml pipet (same as the moles of complex- all the moles in the flask came from the pipet)?
-Concentration of salicylate ion in the volumetric pipet (moles/.001L= conc.)?
-Concentration of salicylate ion in the 250 ml flask (same as the conc. in the pipet- no dilution occurred between the flask and the pipet, so the conc. doesn?t change)?
-Moles of salicylate ion in the 250 ml flask (conc. * .25L = moles)?
-Moles of salicylate ion in the Erlenmeyer flask (same- all the moles in the 250 ml flask came from this flask)?
-Moles of acetylsalicylic acid in the Erlenmeyer flask (same- in equation 1, the salicylic acid: acetylsalicylic acid mole ratio is 1:1)?
-Mass of acetylsalicylic acid in the Erlenmeyer flask (moles of ASA * 180.15g (the molar mass of ASA) = mass of ASA)
 

podany24

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You said you are doing this colormetrically, so do you have a standard curve of your absorbances?
 

felipe5

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woah,,,,,,,,,we did the aspirin lab in organic 1 lab, not gen chem......are we THAT behind here in Kansas? geez, and all this time I thought we only lagged in the evolution department :D
 

Fish3715

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Originally posted by podany24
You said you are doing this colormetrically, so do you have a standard curve of your absorbances?

Yes.

Absorbance = 1520.4(conc of salicylate ion) + .0004
 

Fish3715

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One more question: it seems that you all agree that the water is what caused my >100% yield for the synthesized ASA. However, I also got over 100% yield when I had to colormetrically analyze an aspirin tablet (my partner got ~90.4%). Would water still be the reason?
 
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