posted by ~christina~ on .
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)
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.