The calculations for colligative properties do not depend on the identity of a substance that is being used because only the amount of the substance that is used plays a role. The types of calculations that are colligative properties are: freezing point of depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.1 Colligative properties are present in everyday life, such as adding salt to a boiling pot of water. By adding salt to the water, the boiling point of elevation is raised and the boiling salt water will have a higher temperature compared to the temperature of boiling pure water.2
The colligative property that will be used in this experiment is the freezing point of depression, which can be calculated using the equation: ∆T_f= k_f × m × i In the equation the kf stands for the freezing point of depression constant for a given solvent, m stands for the molality (mol solute/kg solvent) of the solute, and i stands for the van’t Hoff factor of the solute. In this experiment, the freezing point of depression will be calculated for two unknown solutions in order to determine the molecular weights and identities of the unknowns. Experimental For Part A of this experiment, an ice and salt bath was prepared by first filling a 150mL beaker full of ice and weighing out approximately 20g of rock salt to a 50mL beaker.
The ice and rock salt was then mixed together with 20mL H2O in the 150mL beaker and stirred thoroughly with a metal spatula. More ice was then added and a 1” stir bar was placed inside of the beaker. The stirring was then turned on and a glass thermometer was placed inside the ice bath to ensure that the temperature of the ice bath was between -10 and -15oC. The LabQuest was then prepared for data collection by plugging it into a power outlet and then plugging in a temperature probe to the Channel 1 slot. The “Mode Box” was kept to the “Time Based” option, the “Interval Box” was changed to “15” s/sample, and the “Duration Box” was changed to “1200s”.
Clicking the “OK” button then saved these changes. 20.0 mL of deionized H2O was then added to a clean 40mL glass vial, and the vial was then attached to a ring stand above the ice bath. The temperature probe was then placed inside of the vial and the ring stand was then lowered to submerge the vial into the ice bath. The “collect” button was then pressed on the LabQuest to begin data collection in the “Table View” option. While the data collection was occurring, the temperature probe was then used to stir water in the vial in order to collect the change in temperature for the water.
The water was stirred constantly during data collection and after 8 similar temperature readings were recorded, the data collection ended. The vial was then removed from the ice bath and the deionized H2O was then discarded down the sink and the vial was then washed with deionized H2O from the sink and thoroughly dried. For Part B, excess water from the ice bath was removed using a plastic pipette and more ice and rock salt was then added to keep a temperature between -10 and -15oC. Run 2 was initiated in the LabQuest by pressing the “file cabinet” button. Approximately 2.0g of Unknown #2 was weighed using the 40mL glass vial, spatula, and the scale. The final weight of the solution with the vial was 27.698g.
The solid inside the vial was then dissolved by added 20mL of deionized H2O using the bottle top dispenser and then using the temperature probe to stir. The vial was then attached to the metal ring stand and lowered into the ice bath with the temperature probe still inside of the glass vial. The “collect” button was then pressed to begin data collection. The temperature probe was used to stir the solution constantly during the entire duration of data collection. After 8 similar temperature readings, data collection was ended. After data collection, the glass vial was removed from the bath and the unknown solution was then discarded in the liquid waste container.
The glass vial and temperature was then cleaned with fresh deionized H2O and thoroughly dried. The same process for Part B was then repeated for Part C with 5.0g of Unknown #4. After data collection was concluded for Run 3, all the data was then saved onto a USB drive and was checked that it properly saved before the LabQuest was turned off. The temperature probe was then unplugged from the LabQuest and the LabQuest was then unplugged and turned off as well. The solution and ice bath was then discarded by pouring it down the sink.
The temperature probe and all the materials used were then washed with fresh deionized H2O and then dried and placed back to their proper containers. For Part D, a computer in the lab was used to open up the saved data in Logger Pro 3.11. The freezing points of each of the solutions were then determined by analyzing their data points on their graphs. These graphs were then saved on a USB drive and the “corrected” freezing points of each unknown was calculated by subtracting the experimental freezing points from the experimental freezing pint of water.
Results & Discussion The purpose of this experiment was to identify the unknown solutions of two compounds, one ionic and one molecular, by calculating the molecular weights using their freezing points. If the experiment were to be repeated, a suggestion would be to replace the 40mL glass vial with a bigger vial. This would aid in the stirring of the solution during data collection.
Another suggestion would be to place the ice bath inside another beaker only filled with ice to ensure that the temperature of the ice bath stayed between the desired temperature ranges. The molecular weights of Unknown 1 and 2 were calculated in steps 1-3 of the Data Analysis portion by first calculating the molality of each unknown using: ∆T_f= K_f ∙m ∙i The normalized freezing points for Unknown 1 and 2 were (-2.44 oC) and (-7.13 oC) respectively.
A van’t Hoff of 2 and the constant 1.86 oC/m was used for the calculations. Once the molality was calculated, .6559 m for Unknown 1 and 1.91667 m for Unknown 2, the number of mols was then calculated by using the formula: m= (mols solute)/(Kg solvent) After the amount of mols were determined, .01311 mol for Unknown 1 and .03822 mol for Unknown 2, it was then used to determine the molecular weight by dividing the amount of grams over the amount of mols. The molecular weight of Unknown 1 came out to be 152.549 g/mol and the molecular weight of Unknown 2 came out to be 52.174 g/mol.
The known molecular weights of NaCl and KI are 58.44 g/mol and 166 g/mol, respectively. Using these known molecular weights, Unknown 1 was then determined to be KI and Unknown 2 was NaCl. The “theoretical” freezing point was calculated for Unknown solution 2, assigned solution, using the exact amount of 2.649g that was used. Using equations 1 and 2, the amount of mols of NaCl came out to be .0453 mols and the molality came out to be 2.265m. The molality was then multiplied by the constant, 1.86 oC/m, and a van’t Hoff factor of 2, to find the freezing point.
The “theoretical” freezing point of NaCl came out to be 8.4258oC. Using this “theoretical” freezing point and the “corrected” freezing point that was calculated in Part D of the experiment, 9.7oC, the percent error was calculated to compare the two values using the formula: % error= ■(|&(theoretical FP-corrected FP)/(theoretical FP)&| ) × 100% After plugging in the correct values, the percent error for NaCl (Unknown 2) came out to be 15.1226%.
The molecular weights of Unknowns 3 and 4 were calculated in the same way that the molecular weights were calculated for Unknowns 1 and 2 in the Data Analysis. The calculations were found using the normalized freezing points of each unknown, -3.47oC for Unknown 3 and -2.29oC for Unknown 4. Using these values, and 5.0g of the substance; the molecular weights of Unknowns 3 and 4 came out to be 134.006 g/mol and 203.057 g/mol, respectively. The known molecular weight for fructose is 180.16 g/mol and the known molecular weight for sucrose is 342.297 g/mol.
Based on these known molecular weights, it can be inferred that Unknown 3 is fructose and Unknown 4 is sucrose. The same process that was used to calculate the “theoretical” freezing point of NaCl was also used to calculate for the “theoretical” freezing point of sucrose, assigned unknown #4. However, the weight of the sample was 4.692g instead of 2.649g and the van’t Hoff factor used was 1 and not 2. Using this method, the “theoretical” freezing point came out to be 1.274oC.
This “theoretical” freezing point was then compared to the “corrected” freezing point by calculating the percent error, using the same equation as before. Using the formula and the value of the “correct” freezing point found in Part D of the experiment, the percent error came out to be 17.73%. In chemistry, supercooling means that a liquid is cooled below its freezing point without solidification or crystallization. During trials from Parts A-C, supercooling was only observed in Part B and C when dealing with the NaCl and H2O mixture and the sucrose and H2O mixture. In these trials, the solution did not begin to solidify until it was well below the actual freezing point of H2O, which is 0oC.
