organic lab question 1 and 2
Matt was heating his methanol solvent in an E. flask on a hot plate to use for recrystallization. The TA came by and noticed he didn't have a stir stick or boiling chip in the flask and told him to use one, please. Matt dropped a stick in the flask and suddenly the liquid exploded out of the flask. Oops! Tell me why that happened. What should he have done instead?
A) The initial idea of adding a stir stick or boiling chip into a flask before starting the process of recrystallization is to ensure that the liquid doesn't reach a state where it will boil over or potentially explode. It aids in the process of boiling the liquid so that the method is done efficiently, slowly, and properly. What Matt had done instead was that he had added the stir stick into the flask when the liquid was already past the point of being at a stable state. The liquid was near the point of over boiling, and disturbance of the solution thus resulted in an explosion of the entire solution. To ensure that this incident doesn't occur, Matt should have initially added his stir stick to the flask at the beginning of the procedure and before he began the process of boiling the liquid.
Explain why the boiling point of your unknown liquid is at the point when the last little bubble escaping from the capillary tube appears to get sucked back into the tube. A good explanation will 1) explain where all those bubbles escaping were coming from and their likely composition, and 2) surely include the term vapor pressure.
A) Upon heating a liquid to its boiling point, a slow stream of bubbles will begin to come out of the liquid (from the capillary tube). Before reaching the boiling point, bubbles will begin to form from inside the liquid and rise to the top. This happens because once the liquid being determined for boiling point is heated, the air trapped inside will expand and go up through the liquid, thus becoming a gas. When the last of the stream of bubbles escapes from the capillary tube is when the boiling point is reached. The boiling point of the liquid occurs when the vapor pressure of the liquid equals the atmospheric pressure, meaning that equilibrium will result between the atmospheric and vapor pressures, and resulting in liquid rising from within the capillary tube.
Sophia and her friend Nora have the same unknown solid. Sophia has progressed a little faster with the purification process, but now she is standing around twiddling her thumbs because no crystals are forming, even over an ice bath. Nora decides that Sophia's problem is too much solvent. Determined not to have the same problem, Nora uses a minimal volume of hot solvent to dissolve her compound before doing the gravity filtration step. She begins to pour her hot solution into the filter cone of a long-stemmed funnel and ....is that missed impurities sprouting up on the funnel walls? Why so shiny? Explain what Nora did wrong and how to fix the problem.
A) What Nora did wrong was that she used a long-stemmed funnel. By using a long-stemmed funnel, it can cause a problem upon cooling the solution. Once the solution is cooled, crystals will form, but some of the crystals can also form in the long stem of the funnel, therefore blocking anything that can pass through it. To do the procedure correctly, Nora should use a stemless funnel instead of a long-stemmed funnel, to ensure that no crystals that have formed will get stuck in the stem.
Sam knows that he has one of 3 possible liquid unknowns: tetrahyrofuran (bp 65-67°C), ethyl acetate (bp 76-77°C), or toluene (bp 110-111°C). He sets up for a microscale bp determination and clamps the whole shebang in a water bath. Fifteen minutes late the water is boiling like crazy, but he has yet to see a single vapor bubble escape from the capillary tube. He double-checks his set-up and everything is fine so now he's totally confused. What must his unknown be and why? Break it to him gently...he doesn't want to feel like a total idiot.
A) What Sam's unknown was based on the possible liquid unknowns and the way in which his technique was proceeding is Toluene, with a boiling point range of 110-111 C. This could be determined because Toluene has a higher boiling point than water (100 C), and therefore, water will boil before Toluene does. By observing the bubbles that formed and not realizing that it was water instead of his unknown, Sam became very impatient- thinking he had performed his lab incorrectly. What Sam should have instead done was that he should have patiently waited until the temperature of the thermometer went up, past, or close to the actual boiling point of Toluene. After doing this, he will see vapor bubbles escaping from his capillary tubes and can therefore prepare to take the readings for the liquid boiling points.
Taylor was harvesting her beautiful, pure crystals via vacuum filtration from a methanol/water mixed solvent system. She poured the solution containing her crystals into the Büchner funnel. Then she rinsed her flask with some warm methanol and poured that over the crystals resting in the Büchner... oops! Tell what happened and why.
A) What Taylor did wrong was that she added warm methanol into the flask to wash out the crystals. What resulted from this happening was that it dissolved the crystals within the solvent, thus causing them to pass through the Büchner funnel once added. In order to fix this mistake, Taylor should use cold methanol instead of warm methanol to wash out the crystals from the flask, and then pouring them into the Büchner funnel. By doing this, it will ensure that no crystals will dissolve in the warm methanol and pass through the funnel.
PJ's unknown solid 1) dissolves in hot ethanol, 2) is essentially insoluble in hexane, and 3) is insoluble in cold water, but sparingly soluble in warm water. Outline the recrystallization procedure you would suggest she use here.
Based on the list of possible solvents provided, the hot ethanol and cold water mixed solvent set is the best possible solvent to use for the process of the recrystallization of his solid. For this technique, ethanol needs to be hot when it is added to the compound to ensure that the compound will dissolve in the solvent. Place the flask containing ethanol and the unknown compound onto a hot plate, and let it heat while stirring it to ensure that the compound will dissolve properly. Once the unknown is dissolved, the flask should be removed from the hot plate and set to cool to room temperature. Once the flask containing the solution has cooled, the addition of water is necessary to make the solution turn cloudy. Place the cloudy solution into an ice bath and wait for ice crystal formation to take place. If no crystal formation occurs, scrape the flask glass wall to help speed up the crystallization process. Once a sufficient amount of crystals have formed, transfer the solvent-unknown solid solution through vacuum filtration. Let the crystals that have formed on the Buchner funnel dry and settle as much as possible before collecting the recrystallized product and obtaining its melting point.
Haylee was very rushed and still needed the mp of her pure microscale unknown. She filled her capillary tube with the proper amount of sample and placed it in the apparatus. She quickly ran the mp and was dismayed when not only did the observed mp cover a 6°C range, but was also higher than expected. Explain what she did wrong.
Hayley was in a rush while trying to run a particular portion of her experiment. Because she was in a rush, the probability of an error taking place is very likely to occur. What Hayley did was that she ran her melting point (mp) determination, while most likely running her ramp rate too quickly, or improperly, which is not recommended. If this happened, the time to record the initial reading could have already passed, due to the rapid temperature changes and dramatic melting point range thus taking place. If she did not do this and still came to the error as described, frequent and high observation should have been in place during the mp determination. For proper mp determination, Haylee could set the range for start and end temperatures closer to the temperature range close to the ranges of the possible unknowns. If repeating the step in an attempt to repair her error, Hayley could also set the temperature range of the mp determination to one that is closer to the range of the first trial. To get an accurate reading regardless of time constraints, it is crucial for Haylee needs to set the apparatus ramp rate at a rate lower than her initial trial, so that when solid begins to melt, she will be able to obtain a sharper and more accurate reading of the unknown melting point
