MrHomeScientist
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Getting the Most Out of Solution
This is probably a simple problem, but I'm not completely sure how to do the math on it.
My goal is to make 100g NH4Cl. To do this, I am using the reaction:
KCl + NH4NO3 --> KNO3 + NH4Cl
I dissolved 139.25g KCl and 149.53g NH4NO3 in 536mL of distilled water. I chose this amount of water specifically to get the
maximum amount of KNO3 out at my fridge's temperature of 3C. I just pulled the dish out of the fridge and the crystals are drying on the
filter paper now. So, let's say I have X grams of KNO3 that has been removed from solution.
The next step is to boil down the water to a lower level, put it back in the fridge, and gather more KNO3, and repeat this to squeeze as
much nitrate as possible out of the solution. Finally, I'll allow the rest of the liquid to evaporate and collect my pure NH4Cl.
My question is: how much should I reduce the water volume to pull out more KNO3, while leaving everything else still in solution? I've
removed X grams of potassium and nitrate ions, but there's still potential for KCl, NH4Cl, and KNO3 to crystallize. I made a
chart of the various solubilities that I'll attach here.
Maybe I'm overthinking this? I think the potential for KCl crystallizing is what's throwing me off.
Attachment: ammonium nitrate.xlsx (15kB)
This file has been downloaded 364 times
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fusso
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I see the solubility as mass per volume. Shouldn't molarity be used instead?
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happyfooddance
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Try it exactly as you said. I have made kilos of ammonium chloride exactly this way. Some NH4Cl will crystallize with the KNO3 if you boiled it down
too much, or cool too intensely/quickly, but 2 things:
1) it is very easy to distingush KNO3 (long straight needle-like), KCl (square pyramidal), and NH4Cl (fractal, snowflakes) salts by eye, and even
determine relative concentrations by taste.
2) it is fairly easy and fast to get a clean separation, filtering or decanting is easy.
Edit: it sometimes helps to collect fractions by temp, for example, after decanting, putting solution in the freezer to collect more crystals.
Sometimes if it gets too cold the whole thing will turn to a mass of ammonium chloride crystals, so you have to watch it.
So you are right about your concerns but I also think you might be overthinking it.
But because you are aware of these things I think this will be really easy for you.
[Edited on 1-5-2019 by happyfooddance]
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morganbw
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I love using mols, makes life simple. To use molarity, I have always had to convert to grams. That is something my scale can understand, it does not
weigh in mols per se but needs a transformation into grams.
When someone states grams instead of mols, pls be aware that the grams may have been determined from the mol weights.
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r0749547
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try to desolve it in an ammonia/NaOH solution once you have pulled most of the KNO3 out.
it will saturate the sollution with ammonium making the NH4Cl drop out of the solution first.
while the KCl, NH4NO3 and KNO3 will stay in the solution.
and if you want us to do the math, the volume of the solution would be nice 
if my calculations are right... I kind of made a lot of roundings...
you should be able to boil the solution down to about 250ml.
but to be safe I would go to 300ml-400ml.
[Edited on 5-1-2019 by r0749547]
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MrHomeScientist
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I was able to remove 50.8g of KNO3 after cooling. I reduced the solution volume to 350mL, and allowed that to cool. I thought this would
keep everything in solution, but a small amount (maybe 10g) of square crystals formed. Not sure if this is KCl or NH4Cl.
After these had formed, I filtered them off. To my surprise, I returned later to find more crystals had grown, but this time it was a mixture of
needles, squares, and little "snowballs." Now I'm even less sure what's happening! I filtered those off as well, and I'll put the remaining liquid
back in the fridge and see what happens.
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MrHomeScientist
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Well it's definitely a different crystal habit now. Much more snowy, definitely not the pure needles of KNO3 from the first batch. A little
frustrating because there should still be over 100g of nitrate in there! This isn't a great way to make ammonium chloride, it seems. I have a ton of
ammonium nitrate so I was hoping I could use what I had.
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