Recrystallization

[gammagirl]

A student performed successive recrystallizations on an impure mixture where there was a 10% by weight impurity. After one recrystallization, she obtained 80% of the original weight of crystals back. After a second recrystallization,  
she obtained 60% of the original weight of crystals back. After a third recrystallization, she obtained 40% of the original weight of crystals back.  If the original amount of contaminant was only 10%, why did she only obtain 40% of the final pure product?                                                                                                                                                                                                                                                                                            This is what I understand so far, 10g impure so 90 gm pure.........so now there is 80 gm. However,  the 10% impurity may mean that part of the 90 % is contaminated. Can someone push me along?

[Sensei]

Maybe crystals contained water of crystallization?

https://en.wikipedia.org/wiki/Water_of_crystallization

https://en.wikipedia.org/wiki/Hydrate

And after each cycle, it was changing amount of water of crystallization.. ?

 

What solvent student used in the above experiment.. ?

 

 

[gammagirl]

My thoughts exactly. That the solvent amount is reducing, but how does that account for the 20% drop? 1st recrystallization 20 gm impure 80 pure. 2nd recrystallization 40gm impure 60 gm pure. 3rd recrystallization 60 impure and 40 gm pure. The solvent is not mentioned in the problem. What is the math behind this?

Edited September 20, 2017 by gammagirl
clarification

[dylrovertson]

Typing up my organic chem lab report and realizing I still don't know what recrystallization is after I already did the lab. 

Just remember these things

new favorite recrystallization technique: add hexanes, then DCM to dissolve. add more hexanes. heat until all DCM boils off. cool, filter.

[hypervalent_iodine]

5 hours ago, Sensei said:

Maybe crystals contained water of crystallization?

https://en.wikipedia.org/wiki/Water_of_crystallization

https://en.wikipedia.org/wiki/Hydrate

And after each cycle, it was changing amount of water of crystallization.. ?

 

What solvent student used in the above experiment.. ?

 

 

 

Where was water mentioned anywhere in the OP? 

6 hours ago, gammagirl said:

A student performed successive recrystallizations on an impure mixture where there was a 10% by weight impurity. After one recrystallization, she obtained 80% of the original weight of crystals back. After a second recrystallization,  
she obtained 60% of the original weight of crystals back. After a third recrystallization, she obtained 40% of the original weight of crystals back.  If the original amount of contaminant was only 10%, why did she only obtain 40% of the final pure product?                                                                                                                                                                                                                                                                                            This is what I understand so far, 10g impure so 90 gm pure.........so now there is 80 gm. However,  the 10% impurity may mean that part of the 90 % is contaminated. Can someone push me along?

 

Think about the masses you are obtaining each time. How much is being lost in the first, second, and third recrystallisation (assuming an original mass of 100 g)? Is it the same loss each time, or different? Why? Hint: you should assume that the volume of solvent used for the recryst is the same each time. You will also want to think about how recrystallisation works - specifically which property you are exploiting when you perform it.

[John Cuthber]

10 hours ago, Sensei said:

Maybe crystals contained water of crystallization?

https://en.wikipedia.org/wiki/Water_of_crystallization

https://en.wikipedia.org/wiki/Hydrate

And after each cycle, it was changing amount of water of crystallization.. ?

 

What solvent student used in the above experiment.. ?

 

 

But the water of crystallisation in a salt is generally a fixed proportion.

So, for example, each molecule of copper sulphate crystallises from water carrying 5 molecules of water with it. Similarly, sodium carbonate takes 10 molecules of water.

 

So this idea just doesn't make sense.
It's much simpler to expail.

You recrystallise something, but , when the solution has cooled, not all the stuff crystallises out- some is still left in solution.

The more you recrystallise, the more you lose.

 

[gammagirl]

Gentlemen, The vague answer, possibly, is that some crystals are left behind in the solvent during each recrystallization so this causes a decrease in recovery. In addition, successive recrystallizations result in soluble impurities contaminating the filtrate, which reduces the yield of pure crystals. The question is," Is there some math that goes along with this 10 % impurity that results in successive 20% decrease in yield with each subsequent crystallization?"

[hypervalent_iodine]

Not really. Did you have a go at answering the questions I asked in my previous post?

[gammagirl]

Dear hypervalent_iodine, if you start off with a 100gm mass 10gm is impurity 90 gm is pure product. So during the second crystallization, at most 80 gm is the maximum yield. Then, ......

 

[hypervalent_iodine]

I am asking you what mass you lose every time the recryst is performed. They state the first one is 80% recovery of the original mass, so that's a loss of 20%. The second gave 60% of the original mass, so that's another loss of 20% (80-60 = 20). The final recryst is the same story. You started with 60% of your original mass, and end up with 40%, so that's another loss of 20%. Why do you think that the amount lost is the same, when the amount put in is not? Again, the amount of solvent used is assumed to be identical throughout. 

Oh, and you didn't answer my other question. What property are you primarily exploiting when performing recrystallisation? 

[gammagirl]

The property you are exploiting is removal of the impurities. And why is always the same......is the puzzle to me. There are impurities that are soluble and insoluble in the mother filtrate along with the pure product.

[hypervalent_iodine]

1 hour ago, gammagirl said:

The property you are exploiting is removal of the impurities. And why is always the same......is the puzzle to me. There are impurities that are soluble and insoluble in the mother filtrate along with the pure product.

Not really what I was getting at. Or, it sort of is but I don't think you've fully realised it. 

Recrystallisation uses, mainly, differences in solubility in various solvents at various temperatures in order to purify a substance. At high temperatures, you want your compound to be (ideally) fully dissolved. Insoluble impurities can then be filtered off. At low temperatures, you want your compound to crash out of solution, leaving behind soluble impurities. Now, this does not mean that all of your compound will crash out. That depends entirely on the solvent, the temperature, volume, and solubility of the compound. Solubility is not an all or nothing phenomena. Your compound might have a solubility of 100 g / L at a certain temperature, which means that at most, you will only get 100 g dissolved in one litre. If you put 150 g of the same compound in one litre, you will have 50 g of solid and 100 g dissolved. If you put 200 g of compound into one litre, you have 100 g of solid, but still only 100 g dissolved. The amount dissolved won't increase unless you increase the temperature or volume. 

All of which is to say, the reason you always lose 20% of the original mass every time you recrystallise as per the given question is due to solubility. For simplicity, let's pretend that your compound (assume 100 g for the first recrystallising) has a solubility of 120 g / 100 mL at high temperatures, which means when you heat it during recryst with 100 mL it will all dissolve. At the low temperature, let's say it has a solubility of 20 g / 100 mL. So when you cool it, 80 g will crash out and 20 g stays dissolved. When you put the 80 g back through a second recrystallisation, the solubility doesn't change, so when you cool it down, you still have 20 g left dissolved in solution. Do you get what I'm saying? I realise that this might be a bit confusing when you also incorporate the impurity stuff they talk about, but I believe all they really want you to identify is the fact that your compound / mixture is still partly soluble in the cooled solution, hence you lose roughly the same amount every time you do it.

[John Cuthber]

7 hours ago, gammagirl said:

 The question is," Is there some math that goes along with this 10 % impurity that results in successive 20% decrease in yield with each subsequent crystallization?"

In general, no.

[gammagirl]

Only someone really smart could give an answer like that, especially since no solubility data was given in cold versus hot to hint at that issue as the main point. That explanation is the entire basis of recrystallization.

 

Edited September 21, 2017 by gammagirl
clarification