Aiieeee!
Todays lab...good grief.
I was supposed to dissolve 0.3g of impure sulfanilamide in enough ethyl alchohol (95%) and me and my partner did the lab together...sort of.
She went and weighed 0.301g of impure sulfanilamide. Then she put some ethanol in there and I decided that she needed more so I put some more in there (25ml erlenmeyer flask). It was about 3 pasteur pipette squirts of boiling ethanol. I had to add that much because the center of the flask was elevated higher than the sides and pieces would get stuck on the middle as the ethanol evaporated. The solute with the compound was boiling for about 10 sec and alot was dissolved but there were some 1/10th of a centimeter large pieces still not dissolved. Since I was doing another lab to see whether 2 chemicals were miscible or immiscible I was busy. She however decided to not pay attention to the flask. I was notified by her that there was about a centimeter of ethanol left in the flask and was told to get more. I went to get more expecting her to remove the flask (isn't that common sense?), well when I came back I see all of the compound has no solute to be dissolved in anymore and is all evaporated. I put some ethanol in there (3 pipette squirts) (after boiling it) then it got worse.
The next time it was boiling, there was more of the compound that would not dissolve. It wasn't a few pieces anymore but rather about 0.01grams worth of undissolved compound in the solute. The pieces kept having bubbles come from them but nothing happened to indicate that it was being dissolved. I was afraid if I added any more that it wouldn't crystalize later so I didn't add any more since theoretically it shouldn't take 6ml of ethanol to dissolve the compound from what I calculated. =(
Well I called the lab instructor over. He tipped it a bit and swirled it and told us that we had too little solute and I added more since it had evaporated again. Then he looked at it and swirled it. I forget that I kept swirling it during the boiling process anyways I did swirl it. He then left and we were left to boil it on our own. We boiled it for 10 sec and the same ammount as before just wouldn't dissolve. With 20 min left in lab our lab tech told us to go and grab someone else's purified crystals and to use it to find the purified crystal's melting point. I did that while my partner went and tested the impure sulfanilamide for the boiling point.
For the pure crystals assuming they were pure crystals.
I got:
1st drop of liquid=> 163oC
all compound dissolved=> 168.8oC
~Can anyone help me figure out why this happened?
-could it have happened if the spatula my partner used to scrape the sides of the beaker if she forgot to wash it after using it to stir her solution from the previous lab?
(I suspected she didn't wash it but she had dipped it in there before I could say anything) The previous lab's chemicals were mixing
Malonic acid + water
Malonic acid + methanol
Malonic acid + hexane
( I did another part of the lab with the same mixing except not using Malonic acid but rather benzophenone but she used her microspatula to go and scrape down the sides of the flask and also to try to crush some of the sulfanilamide pieces that didn't dissolve)
Could that have caused the pieces to not dissolve?
Another theory would be that we didn't add enough to it but if the lab tech looked at it and we added more ethanol in front of him and it still didn't work I don't think that would be the problem.
Note: it worked for everyone else.
It has gotten me highly stressed since I'm worried that I'll get points taken off since I have nothing to calculate, even thought the lab instructor told us to record what went wrong and go onto the melting point determination. I seriously don't know what went wrong with this seemingly simple lab. =(
Help please
I pass except for two or three comments.
The range in the melting point of the "pure" crystals shows that they weren't that pure OR that the temperature was increasing too fast so that the crystals could not come to equilibrium; i.e., all crystals would have melted at 163 or 164 given a little more time. The fix for the first is crystals of a higher purity; the fix for the second is slower heating or better control of the temperature during the melting point determination phase of the experiment.
As for the solubility of the sulfanilamide, there are too many unknowns to know what happened. How high did the temperature get while the sample was dry? Was it high enough to alter the sample so that after heating part of it was not sulfanilamide? How pure was the sample when you started? If your sample came from the same source as your lab colleagues, we would assume your sample was the same as theirs. You don't know the answers to these questions since you weren't there. Your lab partner may not know the answers if s/he did not pay attention. And trying to diagnose the problems isn't any easier from 2000 or 3000 miles away as I am, and I didn't see what went on either. That's about the best I can do.
As for the solubility of the sulfanilamide, there are too many unknowns to know what happened. How high did the temperature get while the sample was dry?the room temperature was normal and not too hot or cold [I wore a light jacket]) Was it high enough to alter the sample so that after heating part of it was not sulfanilamide?If your refering to the room temp then it's what I said before but If your refering to when I heated it on the hotplate then it was a rolling boil How pure was the sample when you started?I don't know, should it have said so on the label? My book said that usually it had 5% contamination
If your sample came from the same source as your lab colleagues, we would assume your sample was the same as theirs. You don't know the answers to these questions since you weren't there.I'm sort of confused, you first say the solubility then now it sounds as if your talking about the crystalized "pure" sulfanilamide Your lab partner may not know the answers if s/he did not pay attention. And trying to diagnose the problems isn't any easier from 2000 or 3000 miles away as I am, and I didn't see what went on either. That's about the best I can do.I do appreciate any part of this you can help me with Dr.Bob
I don't know if you read that part about the possible contamination of the hexane/methanol/or malonic acid but from what you said does that imply that if I heated the solute enough when dissolving the sulfanilamide then it wouldn't matter?
Since I was doing another lab to see whether 2 chemicals were miscible or immiscible I was busy. She however decided to not pay attention to the flask. I was notified by her that there was about a centimeter of ethanol left in the flask and was told to get more. I went to get more expecting her to remove the flask (isn't that common sense?), well when I came back I see all of the compound has no solute to be dissolved in anymore and is all evaporated. I put some ethanol in there (3 pipette squirts) (after boiling it) then it got worse.
The next time it was boiling, there was more of the compound that would not dissolve. It wasn't a few pieces anymore but rather about 0.01grams worth of undissolved compound in the solute.
From the above, I understand that your lab partner allowed the ethanol-sulfanilamide mixture to evaporate all of the ethanol. That is the temperature to which I refer. We don't know how long it stayed on the hot plate (as a dry sample) nor how hot the hot plate became nor how much damage, if any, it did to the the sample. I wouldn't stress out over this. What's done is done. Pick up the pieces and march forward. As to the malonic acid, I read ALL of the message, but I didn't imply anything about that part. Sorry I can't be of more help.
Alright then I shall move on...
I personally think that I should think about what went wrong instead of just going "oh something went wrong and that's that" b/c I wouldn't learn anything from that Dr.Bob righty?
But thanks for reading it anyways
Dr.Bob =)
You're right, of course; however, you were not with the sample all of the time so what may or may not have happened is speculation and/or second hand information so it will be difficult to draw any kind of firm conclusions. If you could firmly identify the problem then you would know what not to do or what to do the next time but such is not the case. That is why I advised against over analyzing it because you don't have enough information to analyze it. (I know of that which I speak because I'm prone to over analyze things. Had this happened to me I would be doing the same thing you are doing and my friends would be telling me not to over analyze it and to move on. I usually do but reluctantly.) So try to put it out of your mind because you don't want it to take valuable time away from those other things you want to think about.
Well thinking about it your right that I don't know exactly what went wrong so I can't technically learn from it.
I over analyze things too.
I'll move on to other things and I do have many other things to analyze too.
Thanks again Dr.Bob=D
I understand that you're feeling stressed about the outcome of your lab. Let's try to figure out what might have caused the issues you experienced.
Based on the information you provided, there are a few possible reasons why the sulfanilamide did not fully dissolve:
1. Insufficient solvent: It seems like you and your partner may not have used enough ethanol to fully dissolve the sulfanilamide. Adding only 25 mL of ethanol to dissolve 0.3 g of sulfanilamide might not have been enough. Additionally, when you added more ethanol later, it didn't result in complete dissolution either.
2. Insufficient heat and agitation: Dissolving solids in solvents often requires some heat and agitation. Boiling the solution for just 10 seconds might not have been enough to fully dissolve the sulfanilamide. Increased heating and continuous stirring may have helped.
3. Impurities in the sulfanilamide: Impurities can interfere with the dissolution process. If the sulfanilamide you used was impure, it might have made it more difficult for the compound to dissolve completely. This could potentially explain why other groups had success with the lab while you did not.
Regarding your theory about the spatula and the previous lab's chemicals, it's possible that some residue from the spatula or the previous lab's chemicals could have affected the dissolution of the sulfanilamide. However, without specific information about the compatibility of those chemicals and possible cross-contamination, it's challenging to say for sure.
Moving forward, it would be helpful to communicate with your lab instructor about what happened and the difficulties you encountered. They will likely take into account the challenges you faced and provide guidance on how to proceed. Remember that the purpose of the lab is also to learn from the experience, so recording what went wrong and discussing it with your instructor will be valuable in understanding the process better.
Lastly, try not to be too hard on yourself. Mistakes and challenges happen in the lab, and it's all part of the learning experience. Focus on understanding the concepts and procedures, and use this opportunity to improve your lab skills.