Colligative Properties & Osmotic Pressure
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In this experiment, the objective was to observe the effects of some of the colligative properties of solutions. The freezing points of distilled water and saltwater were measured, and found to both be about 0 °C. The osmotic effects of different solutions on a dialysis bag and an egg membrane were observed and it was possible to distinguish between hypertonic and hypotonic solutions. Experiment and Observation: The first step in this experiment was to make an ice water bath. This ice water bath was used to find the freezing point of pure distilled water. A test tube of the distilled water was placed in the water bath and the temperature was recorded every 30 seconds until it remained consistent for five consecutive readings. The pure water test tube was removed and a test tube of a distilled water and salt solution was placed in the ice water bath. Again the temperature of the water and salt solution were recorded for five consecutive readings. Data tables were used to create graphs and find the freezing points of the pure water and of the saltwater solution. The osmotic pressure portion of this experiment began with soaking the dialysis bag in distilled water for 30 minutes.
While this was soaking the raw egg was placed in a jar and vinegar was poured over it until it was covered in vinegar. Initial observations of the egg covered in vinegar were made, and it was set aside. The dialysis bag was removed from the distilled water. One end of the bag was closed with a rubber band and filled 1/3 full with corn syrup. The other end was then closed with a rubber band and it was placed in distilled water. After 1 hour, 2 hours, and 12 hours, observations were made. The egg was left in the vinegar for a total of 24 hours and then removed and placed in corn syrup. Temperature of Distilled Water in Ice Bath Time (seconds) 0 30 60 90 120 150 180 210 240 270 Temperature (°C) 22 9 5 1 -‐3 0 0 0 0 0 Temperature of Saltwater in Ice Bath Time (seconds) 0 30 60 90 120 150 180 210 240 270 300 330 Temperature (°C) 22 14 5 4 3 2 2 1 1 1 1 1 The freezing points of the two samples were found by drawing straight lines over the two portions of the cooling curve and finding where they intersected, as shown in the graphs above.
The following observations pertain to part two of the lab with the dialysis bag and the raw egg. The dialysis bag was approximately ¾ full when observed after 1 hour. It was completely full after 2 hours and it had burst off one of the rubber bands after 12 hours. When the egg was submersed in vinegar bubbles immediately began to form around the eggshell. After one hour there was foam on top of the vinegar and there were many bubbles around the eggshell. It was approximately 25% larger than its original size. After 12 hours the egg appeared about 50% larger than its original size and there were fewer bubbles around the eggshell and less foam at the top. After 24 hours submersed in vinegar the eggshell was semi-‐transparent. It was approximately double its original size and there were very few bubbles present. When the egg was removed the shell felt soft and the egg felt similar to a very full water balloon. When the egg was placed in syrup it was noted that the egg did not submerse fully. Approximately ¾ of the egg was below the surface of the syrup.
The egg began to reduce in size and after 12 hours it was very close to its original size. After 24 hours the egg was its original size. Calculations and Error: There were no calculations that were necessary in this lab experiment. The largest source of error is probably the ability to read the thermometer. The thermometer in the lab kit is a regular glass thermometer with markings on the side. It could not be read any better than one degree, and this limited the precision of the temperature readings. Discussion and Conclusion: In this experiment, the freezing points of distilled water and saltwater were measured and found to both be about 0 °C. It was also found that water moves through a membrane to dilute a more concentrated solution. The portion of the experiment using the dialysis bag was interesting and fun because it gave an idea of what goes on in dialysis treatments. It was neat to observe the supercooling of the distilled water in part one of the experiment. The results of the freezing point of the pure water versus salt water were not as expected but this may be because of the tools used for this experiment.
The thermometer reads to 1 degree so it cannot be more accurate than 1 degree Celsius. Also, it may be that the thermometer was not calibrated properly. The freezing point of the salt water should have been lower than the freezing point of the pure water. It may be that waiting longer for the temperature to drop even lower might have given different results. Or this may not have been achievable with a home-‐made ice water bath. To achieve better results, a better quality thermometer would be needed, and maybe a colder ice bath. The ice bath might have been colder if a lot of salt was put on the ice, using the colligative property of freezing point depression. Questions: Part I: Colligative Properties A. Record your observations and your time and temperature data in tables. Use one table for the pure water and one table for the salt solution.
Please see Data Table section above for tables. B. Record the freezing point of the pure water and the freezing point of the salt solution? The freezing point of the pure water was 0°C and the freezing point of the salt solution was 1°C. I do not believe this was the true freezing point of the saltwater solution because it was expected to be lower than the pure water freezing point. C. How do these freezing points compare? The freezing points of the pure water and the salt water have about a 1°C difference between them. D. What are some practical applications of freezing point depression, boiling point elevation and vapor pressure lowering? A practical application of freezing point depression is using salt or magnesium chloride to melt the ice on the roads when it snows. A practical application of boiling point elevation is the use of antifreeze in the car radiator. A practical application of vapor pressure lowering is when a product is freeze-‐dried. Part II: Osmotic Pressure A. To what biological structure is the dialysis bag comparable? How is it similar?
How is it different? The dialysis bag is comparable to a cell membrane. It is similar in that it can regulate the passage of certain substance into and out of bag like a cell membrane can regulate the passage of certain substances into and out of the cell. It is different in that the cell membrane is part of a living cell and has much more complex mechanisms that control what can enter and leave the cell. F. In your experiment is the syrup hypertonic or hypotonic to the egg? In my experiment the syrup is hypertonic to the egg. G. At 23.6°C, 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? Molecular mass can be found according to the following equation Π = MRT. The following calculations were performed to solve this problem: Conversion of 8.34 mmHg to atm: 8.34 mmHg * 1 atm/760 mmHg = 0.011 atm Conversion of 23.6°C to K: 23.6 + 273 = 296.6 K Calculate Molarity using gas constant R = 0.0821 (L atm)/(mol K): Π = MRT gives M = Π/(RT) M = 0.011 atm/(0.0821 (L atm)/(mol K) * 296.6 T) = 4.52 x 10-‐4 mol/L Calculate moles of antibiotic given 0.500 L of solution: 4.52 x 10-‐4 mol/L * .5 L = 2.26 x 10-‐4 mol Solve for molecular mass given 0.302 g: 0.302 g/2.26 x 10-‐4 mol = 1336 g/mol