# Dr.Bob (about finding the ml of ethanol)

posted by .

~From before since the question's been bumped to the next page. Just copied and paste with added stuff.

I'm confused as to what to do for the calculations for my lab.

Lab: Semimicroscale crystalization-erylenmyeyerflask and Hirsch Funnel

Weigh 0.30g of impure sulfanilamide and then transfer to a 10ml Erlenmeyer flask. To a second Erlenmeyer flask add about 6ml of 95% Ethyl alcohol and a boiling stone.

summary of procedures:
Add enough hot solvent to just cover crystals and then heat untill boiling and swirl to dissolve crystals.
To crystalize allow flask to cool slowly. Use ice bath to induce crystalization. Filter using Hirsch funnel. Rinse with 1ml portion of ice cold ethanol. Transfer crystals to a preweighed watch glass.

A pre lab Q..
Calculate how much 95% ethanol will be required to dissolve 0.3g of sulfanilamide at 78%.

How would I find this?? I haven't any clue and there are no sample calculations in my lab book coincidentally.

How would I then find the percent yield?
~ for this wouldn't I go and divide
the (amound left)/(original ammount)*100= percent yield?

I seriously need help on these calculations (they really need to include sample calculations in this book)

Thanks
_______________________________________
The Merck Index lists the solutility of sulfanilamide as "1 g dissolves in about 37 mL alcohol." And do I suppose 78% is actually 78o ?
You can set up a ratio or do it by dimensional analysis. Percent yield is done as you have suggested. They may list the solubility in the book.
_______________________________________
Hm..I found a similar question online but it didn't explain how to go through the question just looked up the solubility of sulfanilamide which is at 78 degrees(210mg/ml) [note I put % before by accident]

Now I'm confused as to what to do with that..=(

I think what's getting to me is the % ethanol and the temperature.

For a normal lab would I be using the 78% as the solubility temperature or some other temperature?

Would the strength of the alcohol matter or not Dr.Bob?
or would I just use the general merck index information as is?
_______________________________________

MOST ethanol used in the labs is 95%. Ethanol forms an azeotrope when it distills and the stuff that comes over is 95% ethanol and 5% water. So don't be concerned with that bit of data. Also, don't worry about the 78 degrees C. Most recrystallizaton experiments call for heating the solvent to near boiling (at least to an elevated temperature) to dissolve the material, then letting it cool and recrystallize. If the solubility is 210 mg/mL. So you want to dissolve 300 mg.
1 mL x 300/210 = ?? mL. Not much is required to dissolve 300 mg. I did a lot of this recrystallization by trial and error, especially when unknowns are the order of the day and I didn't have a clue what I had. I would add the 300 mg sulfanilamide to the flask, add about 1 mL alcohol, heat it up to near boiling, dissolve what will, then add a LITTLE more, usually dropwise (I count about 20 drops to the mL) and reheat and reswirl. If it goes in solution with a little less than what you have calculated, good. By the way, IF a procedure requires something better than 95-5 alcohol, it will say absolute alcohol which means it is 100% ethanol and no water is in it. That is a good deal more expensive than the 95-5 stuff.
_______________________________________

I'm trying to get what you did b/c it seems simple.

For the solubility of sulfanilamide:
At another site from a label of the bottle of the chemica.

Solubility:
7.5 gm/100gm water at 25C (77F).

what is gm exactly? I looked up gm and all I got was GM the car company

what I found before for solubility was:

Solubility of sulfanilamide:
210mg/ml at 78o

So based on this you said to not worry about the temperature. Does that mean that the solubility at a higher temperature wouldn't matter?
Wouldn't it be more soluble at a higher temperature Dr.Bob?

You said:
If the solubility is 210 mg/mL. So you want to dissolve 300 mg.
1 mL x 300/210 = ?? mL

I'm not sure I get this..lets see what I think....

solubility of sulfanilamide: 210mg/ml
I have in the problem: 0.03g

0.30g(1000mg/1g)(1ml/210mg)= 1.428ml
(1.43ml with significant figures)

I guess I got confused when you didn't put in the units.

~But when I do this in the lab I would just use the solubility at 78o Celcius then
(210mg/ml) for my calculations Dr.Bob?

Thanks Dr.Bob =D

so the solubility of

• Dr.Bob (about finding the ml of ethanol) -

I'm not sure I can remember everything from above, but I thought you had looked up the solubility of sulfanilamide as 210 mg/mL. If that is so, then
1 mL x 300 mg/210 mg = about 1.5 mL alcohol.
You are rather certain that it will take <6 mL for that is what the lab manual tells you to measure out and bring to a boil. So-- do as I said before. Add about 1 mL of the heated alcohol to the 300 mg sample of sulfanilamide, swirl, add another few drops, swirl, and continue until all of the 300 mg sample is dissolved. Then crystallize. If you count drops, then about 20 drops is 1 mL; therefore, it should dissolve with about 10 more drops than the 1 mL you started with (assuming the 210 mg/mL is accurate.) It would help if you could determine a number for the solubility of the stuff so you would feel confident that you weren't added far too much ethanol. Adding too much will affect the yield.
How to calculate yield. The same as you do percent anything.
%yield = (amount recovered/0.300)*100.

• Dr.Bob (about finding the ml of ethanol) -

I'm really confused as to finding a RELIABLE source to find the solubility of sulfanilamide.

Do you know at what temperature are they refering to for the Merck index's

1g in 37ml of alchohol?

The reason I'm doubting the 210mg/ml is that I got that from an exact question same question answered on that answering site with a y-hoo in it. The question just had a 0.1g instead of 0.3 like I did. The person answering them posted a link to a powerpoint file. I downloaded that and I saw that it was for the same labs that I was doing and had that listed as the solubility.

However I can't back that up with anything on the web about the solubility of sulfanilamide. I looked but can't find the solubility for some reason and I still don't know what the unit gm means for the only solubility of sulfanilamide that I found or I would be able to convert it and see if it was accurate and matched with the powerpoint slide #'s.

I found solubility on a website as:

Solubility:
7.5 gm/100gm water at 25C (77F).

You didn't say whether you knew what that unit was Dr.Bob...like I said before all I found was GM the company

• Dr.Bob (about finding the ml of ethanol) -

gm is an old abbrev of GRAM. That's the way we wrote it in the good old days. :).
Let me look for the solubility and I'll get back.

• Dr.Bob (about finding the ml of ethanol) -

That clears things up alot Dr.Bob
=D

And I'll check later to see what you find.

• Dr.Bob (about finding the ml of ethanol) -

• Dr.Bob (about finding the ml of ethanol) -

THANKS ALOT for spending all of that time looking and I appreciate the effort it took to look up all that.

Thanks Again Dr.Bob =D

• Dr.Bob (about finding the ml of ethanol) -

Do you have a lab text book? Perhaps it is written by Pavia or others. In many text books there is a graph of the solubility of sulfanilamide vs temperature for 95% ethanol... Look at the graph and determine the solubility at 78 deg C. Hint - it is bigger than 200 mg/mL.
To find how many mL are needed you need to dissolve it you should multiply the inverse of the solubilty times grams to get mL.

• Dr.Bob (about finding the ml of ethanol) -

210 mg of sulfa dissolves in 1 ml of alcohol @ 78 degrees celsius.

(1ml alcohol/ .21g sulfa) 0.30g sulfanilamide = 1.42 ml alcohol to add to sample.

• Dr.Bob (about finding the ml of ethanol) -

I have this lab text book. If you read the question carefully it says use information from the introduction to answer your question. If you then read the introduction very, very carefully, it sends you from page 23 to page 600-something where there is a solubility-vs-temperature graph for temperature and ethanol. That is where the solubility of 210 mg/ml @ 78 C is given and can be verified. It would have saved you some time, but like I said ... they did a good job of hiding the information.

• Dr.Bob (about finding the ml of ethanol) -

Using the volume 1.5mL of 95% ethyl alcohol that's required to dissolve 0.3g at 78 C, how do i find how much sulfanilamide that will remain in the mother liquor after the mixture is cooled to 0 C?

• Dr.Bob (about finding the ml of ethanol) -

Since the solubility of sulfanilamide at 0 C is 14mg/mL, do I multiply 1.5mL and 14 to get how much sulfa will remain in the mother liquor?

• Dr.Bob (about finding the ml of ethanol) -

Calculate how much 95% ethyl alcohol will be required to dissolve 0.75g of sulfanilamide at 78 degrees Celsius.

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