Purpose
The purpose of this laboratory was twofold. Firstly, we aimed to understand the contrast between the freezing points of a pure solvent and a solvent in a solution with a nonvolatile solute, and to compare the two. Secondly, we sought to observe osmosis to gain insight into how dialysis functions.
Procedure
- Make a water bath assembly by doing the following: a. Half-fill the 100-mL beaker with cool tap water.
- Place crushed ice in the beaker so the water level is just below the top of the beaker. The water level should not be higher than the length of the test tube. Sprinkle a little salt into the ice water. Mix well. Half-fill the test tube with distilled water. Set the tube into the 24-well plate. (The well plate will function as a test tube holder. )
- Insert the thermometer into the test tube and take readings every 30 seconds until the readings remain constant, then record the temperature of the distilled water.
- Place the test tube in the beaker‘s ice water bath and set your stopwatch at zero.
- Carefully stir the water in the test tube with the thermometer and record the temperature of the water at 30-second intervals. You should see the temperature of the water rapidly decrease to from -1 C to -3 C then rise to 0 C. At that time the readings should remain constant before again decreasing. This is super cooling.
- Caution: Do NOT let the water in the test tube freeze completely or the thermometer may break.
- Once five consecutive readings have been made at a constant temperature, remove the test tube from the bath and empty it into a sink.
- Refill half of the test tube with room temperature distilled water (at least 10 C) and add 1/8 teaspoon of salt to the distilled water in the test tube. Mix well until dissolved.
- Either prepare a fresh water bath or add more ice and a little salt to the existing water bath.
- Repeat Steps 2 – 5 above using the saltwater solution prepared in Step 7 above. You may not observe super cooling this time.
- Pour the water from the test tube and from the water bath down the drain. Clean up your equipment and replace it in the LabPaq.
Two graphs were created for each of the two experiments. The temperature recordings were plotted on the y-axis, and time was represented on the x-axis in 30-second intervals. On each graph, two straight lines were drawn connecting the initial temperature to the coldest temperature observed, and extending straight across the five constant temperature recordings. The intersecting point of these lines was recorded as the freezing points of both the pure solvent and the solution.
In the second part of the experiment, a glass bowl was filled with distilled water. Dialysis tubing was placed in the bowl and left for 30 minutes. The tubing was then removed from the water and the water was discarded. One end of the tubing was sealed tightly with a small rubber band. Clear Karo syrup was poured into the tubing using a small funnel. The other end of the tubing was closed tightly with another rubber band, similar to how it was done before.
The dialysis tubing was immersed in a bowl filled with distilled water and closely monitored for 5 hours. Simultaneously, a raw egg was placed in a pint jar and observed. The egg was then covered completely with white vinegar, sealed in the jar, and observed again after 12 and 24 hours had elapsed, with the observations meticulously recorded.
The egg was initially removed and transferred to a different pint jar filled with Karo syrup for 24 hours. Subsequently, the jar was sealed and an observation was made and noted. Observations were also recorded after 12 and 24 hours had elapsed. Finally, after 24 hours, the egg was taken out of the syrup, rinsed with tap water, and further observations were made and documented.
Observations
During the second part of the laboratory, osmosis resulted in the syrup from the dialysis bag mixing with the water, and water entering the dialysis bag until they reached a state of equilibrium. I personally do not believe that true equilibrium was achieved because the osmotic pressure led to the rubber band detaching from the end of the dialysis tubing. As for the experiments involving raw eggs, I speculate that a specific element from the eggshell reacted with another element in vinegar, resulting in the formation of bubbles and ultimately causing the disappearance of the shell.
The vinegar caused the egg to become larger as it was hypotonic to the egg. On the other hand, the syrup solution caused the egg to shrink as it was hypertonic to the egg.
The dialysis bag can be compared to a biological structure. It shares similarities with the cell wall as it enables water movement for osmotic equilibrium. However, it differs from cells in its selective permeability based on dialysis tubing size, while cells utilize both active and passive transport mechanisms for substance exchange.
When a cell is placed in a salt solution with a lower salt concentration than the cell, it becomes hypotonic. This leads to water moving into the cell and causing it to burst or undergo cell lysis. On the other hand, if a cell is placed in a salt solution with a higher salt concentration than itself, the solution becomes hypertonic. As a result, water moves out of the cell into the solution, leading to cellular death through crenation or shrinkage.
Problem for Lab Report: At 23.6C, 0.500 L of a solution containing 0.302 grams of an antibiotic has an osmotic pressure of 8.34 mmHg. What is its molecular mass?
8.34 mmHg x 1 atm/760 mmHg = 0.01097 atm
0.01097 atm = M(0.0821) (273 + 23.6)
0.01097 atm = M(0.0821) (296.6)
M = 0.01097/ (0.0821)(296..6) = 4.X10^-4
Molarity = moles/L
4.X10^-4 = moles/0..5X=2.X10-^moless
p>!!o302 grams /2.x10-^moles ==>1342x.g/mols
Conclusion
During the initial phase of this lab, unexpected results were obtained. Initially, I had expected that the freezing point of the salt-water solution would be lower than that of distilled water. However, to my surprise, the rubber band came off from the end of the dialysis tubing, indicating a significant osmotic pressure. Additionally, an interesting observation was made regarding the dissolution of eggshell in vinegar. Despite being mentioned in the lab manual, I did not anticipate this outcome. Vinegar contains acetic acid which breaks down the solid calcium carbonate crystals that make up the eggshell into separate calcium and carbonate components.
The calcium ions are free-floating, while the carbonate forms carbon dioxide. The observed bubbles were actually carbon dioxide being released. I anticipated the egg would grow in size when placed in vinegar and shrink in syrup. I believed I had a good understanding of what would be hypotonic and hypertonic until the experiment’s conclusion. I expected the rinsed egg to increase in size again. In truth, it may have if submerged in tap water for a longer duration.
