this is part of a theoretical lab question, i do not get it. Q.Even if two compo
ID: 685939 • Letter: T
Question
this is part of a theoretical lab question, i do not get it.Q.Even if two compounds have identical solubility profiles in asolvent, it is possible to effect some purification by a singlerecrystallization. Assume you have a 10 g mixture of A and B in a9:1 ratio of A to B and that both have a solubility at roomtemperature of 0.05 g/mL. If you used 20 mL of solvent forrecrystallization, how much B would be expected to be in thecrystallized material? Hint: calculate how much of A and how muchof B would be expected to remain dissolved in the solvent aftercooling to room temperature. (This calculation assumes an idealrecrystallization that probably wouldn?t be totally true in a realexperiment.) Then calculate the amounts of A and of B in therecrystallized solid. What is the ratio of A:B now? and also, What is the purpose of a hot filtration in a recrystallization?When would a hot filtration step not be necessary in a purificationby recrystallization? What experimental observation of the hotsolution would let you know that it was not needed?
Explanation / Answer
Assume you have a 10 g mixture of A and B in a 9:1 ratio of A toB and that both have a solubility at room temperature of 0.05 g/mL.If you used 20 mL of solvent for recrystallization, how much Bwould be expected to be in the crystallized material? Hint:calculate how much of A and how much of B would be expected toremain dissolved in the solvent after cooling to room temperature.(This calculation assumes an ideal recrystallization that probablywouldn?t be totally true in a real experiment.) Then calculate theamounts of A and of B in the recrystallized solid. What is theratio of A:B now?
In 10g combined of A and B in 9:1 ratio you have 9g of A and 1gof B.
Given the solubities 0.05g/ml, 20ml will “hold” 20 x0.05 = 1g. This will hold all of the dissolved B (so no Bcrystallizes), and 1g of A will remain dissolved as well (theremaining 8g of 9g A total will crystallize) So you have“pure” A crystals, although the solution is now a 50/50mixture of A and B.
and also, What is the purpose of a hot filtration in arecrystallization? When would a hot filtration step not benecessary in a purification by recrystallization? What experimentalobservation of the hot solution would let you know that it was notneeded?
Since most solutes have greater solubility at highertemperatures, “hot” filtration is a way to retain moreof a solute in solution, carrying it with the filtrate“through” the filter (which will trap dirt and/orother undisolved impurities). Filtering at a lower temperaturewould have trapped some of the solute (as crystals) in the filter,with a lower amount passing on with the filtrate.
Hot filtration is used to “clean” crystals fromresidues or other impurities which have “dried” on ormixed with the crystals. It can of course also be used toseparate two solutes with differing solubitiles where one of thesolutes is actually less soluble at high temperatures than at alower temp.
You can tell whether hot filtration is needed by attempting todissolve the crystals in a solute. If (after a certain point)adding more solute and/or heating the solution fails to cause moreof the “crystals” solute to dissove, then you canpresume that some (insolubile) impurities are present, whichfiltration can remove. Keeping the solution “hot”ensures that the dissolved solute doesn’t recrystallize andget trapped in the filter with the insoluble impurities.
If heating the solution causes all the solid to dissolve, thenhot filtration isn’t going to do anything. (of course,you could try using a different solute which might give a differentresult). Hot filtration is somewhat dangerous in that thesolution could become flammable at higher temperature, and is alsoprone to “bumping” (violent boiling), particularly whenit is near its boiling point and the solution is close tosaturated.
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