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24 Cards in this Set
- Front
- Back
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Why does a recrystalization give a more pure solid sample? |
- because it is more structurally favourable for the compound to reconfigure in its pure form |
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In the Part1 solubility test, what three solvents will be tested? |
Water, ethanol, ethyl acetate |
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In the Part 1 solubility test, how many mg of compound and how many mL of solvent will be used? |
- ~1mL of solvent - ~75mg |
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Is 3mg/mL considered very soluble? |
No. The high solubility range is 50mg/mL or higher. It is of a very low solubility (insoluble is less than 2mg/mL). |
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Show how to make a concise solubility data table logically arranged into the minimum numbers of rows and columns. That is, give the titles for the rows and columns. |
(See table 1 from results sheet) e.g. temp is cold, 25 degrees C, and hot for each of the three compounds |
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Why is it necessary to remove the hose from the side arm filter flask before turning off the water tap? |
- because the water will go into the funnel (see youtube video) |
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Why should an empty second filter flask be connected between the water tap and the filtration flask? |
- the empty second flask serves as a "trap" to prevent trap water from backsucking into the filter flask, backsucking is caused by changes in water pressure and is unpredictable |
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Draw the solubility vs. temperature profile needed for an ideal solvent for a single solvent recrystalization |
(curve in protocol) - with an increase in temperature, solubility increases |
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Salicylic acid has the following solubility profile in water: 0.2g/100mL@20 degrees C and 7.5g/100mL@100 degrees c. Would water be a good solvent for a single-solvent recrystalization of salicyclic acid? |
Yes, this is a good range, because the ideal is low solubility at low temperatures, and high solubility at high temperatures. |
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Acetyl salicylic acid has the following solubility profile in ethanol: 18g/100mL at 0 degrees C and 80g/100mL at 50 degrees C. Would ethanol be a good solvent for single-solvent recrystalization. |
No, because it is highly soluble at all temperatures. |
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Sucrose has the following solubility profile in water: 200g/100mL at 25 degrees C and 260g/100mL at 80 degrees C. Would water be a good solvent for single-solvent recrystalization. |
No! Sucrose is extremely soluble at all temperatures. |
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For the Part3 procedure, calculate how much additional water should be needed to dissolve the 3g acetanilide in 25mL water at 90 degrees C given that the solubility of acetanilide is 4.3g/100mL in 90 degrees C water. |
100mL/4.3g = x/3g 3g*100mL/4.3g = 69.77mL 69.77mL - 25mL = 44.77mL (45mL) |
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Why is not possible to recrystalize ASA using a single-solvent recrystalization from water, as is done with acetanilide in Part 3? |
Since ASA is a highly polar molecule, it is highly soluble in water. ASA possesses a high solubility across a broad range of temperatures. For the Single Solvent Recrystalization method to work, the compound must be highly soluble in the solvent at high temperatures, and have low solubility in the same solvent at low temperatures. Because ASA is still highly soluble at low temperatures in water, this method is not appropriate. Acetanilide, on the other hand, has a solubility of 4.3g/100mL at 90 degrees C (near the highly soluble range), and 0.5g/100mL at 20 degrees C (at the bottom end of the low solubility range), making it an ideal compound to undergo single-solvent recrystalization in water. |
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What solubility is needed for the first and second solvents used in a two-solvent recrystalization? |
- firstly, the two solvents must be miscible with each other at all temps used in the recrystalization - the solubility must be high in the first solvent and the compound must be insoluble in the second solvent. This is so that the compound can dissolve in the first solvent, and then reconfigure in its pure form in the second solvent as a solid crystal. |
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Which of the following solvents is miscible with water at all temperatures: ethanol, dichloromethane, acetone, ether? |
- ethanol (is an alcohol), acetone |
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Why must a couple boiling stones be added to the flask before heating the acetanilide solution to its boiling point? |
- the boiling stones release oxygen bubbles, ensuring that the heating is uniform. Otherwise, the solution might splash out of the flask. |
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What is the problem with adding too much solvent to a recrystalization, ie much more than the minimum amount needed to dissolve the solid sample? |
If you add too much solvent, you increase solubility, or the distribution of the original compound being dissolved in the solvent. This will make it much harder for the compound to later reconfigure in its crystal form. |
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What is charcoal used for? |
- charcoal removes soluble impurities from the solution by means of adsorption. |
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Why must the solution be ~5-10 degrees celsius below the boiling point before adding charcoal? |
So that the charcoal does not splash out of the flask. |
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Why is it helpful to add celite to the charcoal containing solution before the hot filtration? |
- charcoal is very fine and can be hard to remove through filtration. A filtering agent, such as celite, is added to a solution, and will adsorb the charcoal and make it easier to filter the charcoal out |
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Why must the hot filtration be done as a gravity filtration instead of using a suction filtration? |
- vacuum filtrations cool the solution too much, and crystals are lost on the filter paper and funnel (crystals will start to precipitate out in-process) |
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In the hot filtration, why is the filter paper wetted with hot solvent? |
If the filter paper is not wetted with hot solvent, the solution will be passed through a cool/room temperature structure, which could cause the compound to precipitate as it is being filtered, and get left behind on the filter paper. |
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In the hot filtration of the acetanilide solution, why does some acetanilide precipitate out in the stemless filter funnel before passing through the filter paper? |
If the filter paper is not as hot as the solution, some of the acetanilide that comes into contact with it as the solution passes through might cool down enough to precipitate out in the stemless filter funnel before passing through the paper. |
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In the hot filtration of acetanilide, why must the paper be rinsed with a few mL of hot water? |
- Prior to passing the solution through the filter, we wet it with hot water to minimize the amount of precipitation. Additionally.. - Rinsing the paper with a few mL of hot water not only serves to transfer residual solution left behind in the E-flask, but also will dissolve any white solid acetanilide that precipitated out in the filter funnel before going through the filter paper. |
