Determining the Freezing Point: Lab Experiment

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The purpose of this lab was to determine the freezing point of pure water, the experimental van’t Hoff factor for a substance and the molar mass of an unknown solution.

25mL deionized Water
¼ to ½ inches of rock salt
25mL Sodium Nitrate (NaNO3 )
25mL unknown solution – handle unknown solution carefully

For part 1, the freezing point of deionized water was determined. An ice bath was prepared by filling a 600mL beaker ¾ full of ice and covered with ¼ to ½ inches of rock salt. The ice-salt mixture was stirred with a stirring rod and the mercury thermometer was used to assure that the temperature dropped to -5O C or below. A large test tube was filled halfway with deionized water and placed in the ice bath. The temperature probe was placed in the middle of the water layer and “collect” was pressed in Logger Pro. The water was gently stirred with the temperature probe, making sure to keep the probe from touching the bottom of the test tube. Once the recorded temperature began to level off, the results were recorded as the freezing point of pure water. The value was recorded in a lab notebook. For part 2 of the experiment, the van’t Hoff Factor of sodium nitrate was determined. 25mL of sodium nitrate was obtained and the concentration information recorded. A large test tube was filled 1/3 full of the solution and placed into the ice bath. The probe was placed in the test tube and the “collect” button pressed. The solution was stirred and once the temperature leveled off, the freezing point was recorded. The solution was then thawed. This process was repeated two more times and the average freezing point temperature relative to the measured freezing point of pure deionized water was recorded. The actual and experimental van’t Hoff factor (i) was calculated using the average freezing point and then compared to the theoretical i value. For part 3, the determination of the molar mass of an unknown was determined. An unknown...
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