CH 337: Unit 2
Quiz yourself by thinking what should be in
each of the black spaces below before clicking
on it to display the answer.
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| show | no detectable impurities
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| show | Solids
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| show | recrystallization involves the preparation of a hot saturated solution which is then allowed to cool
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| What occurs as a hot saturated solution gradually cools? | show 🗑
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| In order to obtain a purer sample what must be true about the impurities? | show 🗑
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| show | Slow cooling promotes slow crystal growth which promotes the formation of larger, pure crystals. Rapid cooling promotes rapid crystal growth promotes the formation of smaller, less pure crystals.
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| What useful adage can guide one during the trial-and-error process of solvent selection? | show 🗑
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| As the number of functional groups present increases, solubility increases | show 🗑
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| show | Methanol and ethanol (water is too polar)
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| What two low-polarity solvents are commonly used for recrystallization? | show 🗑
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| show | These solvents are very flammable
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| What is the most crucial aspect of recrystallization? | show 🗑
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| What characterizes an effective recrystallization solvent? | show 🗑
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| What rule of thumb should be followed in order to select an Erlenmeyer flask of appropriate size? | show 🗑
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| In the absence of specific solubility data what rule of thumb should be followed in order to select an appropriate quantity of solvent? | show 🗑
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| What characterizes the manner of solvent addition? | show 🗑
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| show | hot gravity filtration
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| show | when the solute doesn't crystalize from the cooling saturated solution
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| show | a) too much solvent used
b) crystallization during hot, gravity filtration
c) vacuum filtration preformed prior to complete crystallization
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| For how long should one dry a produce that has been recrystallized from a high boiling-point solvent? | show 🗑
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| What is the 1st step to judge if a solvent will be suitable to use as a recrystallization solvent? | show 🗑
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| What is the 2st step to judge if a solvent will be suitable to use as a recrystallization solvent? | show 🗑
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| What is the 3st step to judge if a solvent will be suitable to use as a recrystallization solvent? | show 🗑
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| show | 1) preparation of a hot, saturated solution at the solvent's boiling point followed by removal of any insoluble impurities
2) gradual cooling of the hot, saturated solution
3) collection of crystalline material by vacuum filtration
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| show | a) the use of creased weighing paper
b) the use of creased weighing paper and powder funnel
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| Why is it advisable to set aside a small amount of the crude material before preforming a recrystallization? | show 🗑
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| show | Yes. The advantage to cooling in an ice-water bath is that it facilities maximum crystallization
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| What actions could one take in the event that no material crystalizes after 15 minutes of cooling in an ice-water bath? | show 🗑
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| show | Vacuum filtration
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| show | To prevent the flow of water from the water aspirator to the filter flask
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| Why should one pull some of the recrystallizing solvent through the filter paper, by application of the vacuum source, prior to filtration? | show 🗑
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| show | Volume of water at 100C required to dissolve. 1.2g x 15 mL/g = 18mL
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| How much salicylic acid will crystalize when the solution prepared above is allowed to cool to 25C? | show 🗑
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| show | 1.16/1.20 x 100% = 96.7%
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| show | ice-bath (lowers temp)
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| Assuming 38.2 mL of water was used in the preceding questions, what is the maximum percent recovery of salicylic acid that the solution was cooled to 25C? | show 🗑
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