Freezing Point Depression

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1 Name: Date: Lab Partners: Lab section: Freezing Point Depression The addition of salt to water lowers the freezing point of water. This is the mechanism by which roads are cleared of ice in the winter in snowy climates. This is also the same trick used in making home-made ice cream since ice cream solidifies below 0 C. Many salts and solutes will change the properties of the solvent in which they are dissolved. This phenomenon is called colligative properties of a solution. Most often, colligative properties take the form of lowering the freezing point of the solvent or raising the boiling point of the solvent. One of the features of colligative properties is that the change in freezing/boiling points is often independent of the exact solute but rather the moles of the solute. In this experiment, we will determine the freezing point of water with and without the addition of salt. The objective is to demonstrate the freezing point depression of water with the addition of a known amount of salt. The second part of the experiment is to determine the molar mass of an unknown salt by observing the freezing point depression and using that data to calculate the moles of an unknown salt. Materials needed: 250 ml beaker Digital thermometer Stand & clamp Magnetic stir bar Stirring plate 100 ml graduated cylinder Forceps Glass stir rod Crushed ice Part A Freezing Point Depression of Water with Sodium chloride Experimental procedure: 1) Obtain a clean 250 ml beaker, a digital thermometer, a stand with a clamp, a magnetic stir bar & stirring plate. 2) Weigh the empty 250 ml beaker and record this value on the data sheet. 3) Weigh out approximately 11.6 g of sodium chloride in a weighing dish and put it aside. Record the exact weight of the salt on your worksheet. 4) Get 100 ml of pure (de-ionized) water and add it to the beaker. 5) Add enough crushed ice to bring the water level in the beaker to approximately 200 ml. You should have about 200 ml of a slushy ice/water mixture. 6) Put the stir bar in the beaker and place it on the stir plate. 7) Turn on the stir plate stir setting to about 4. DO NOT turn on the heat; it will heat up the solution too fast and you will get poor results. The stir bar is most efficient when it is

2 centered on the stir plate, so center the beaker in the middle of the plate. Make sure that the stir bar is rotating in circles and not just bouncing around. 8) Clamp the thermometer in the stand and have the tip of the thermometer dangling in the beaker about at the 100 ml mark. You do not want the thermometer to touch the sides of the beaker. 9) Wait until the temperature reaches freezing point and record the temperature on the data sheet. 10) Pour the sodium chloride into the ice-water beaker. The temperature should begin to drop a little more. 11) Make sure the ice is well agitated by the stir bar and it is free to move around as a slushy mix. If there is not enough agitation, then the ice may re-freeze together into one larger mass of ice, which is not as efficient at exchanging heat with the aqueous phase. If the ice starts to freeze into a single mass, then break it up with a glass rod so it can move freely in the water. 12) Wait for most of the ice to melt. The best results are obtained when there is about 50 ml of a slushy ice layer left in the beaker (measured when the stir bar is turned off). You want to have most of the ice melt in order to get a good estimate of the molarity of the solution. The volume taken up by the ice represents volume that does not count in the molarity calculation, thus you want to minimize it. Conversely, do not wait for the last few ice crystals to melt. When there are only a few ice crystals left, the ice cannot keep the solution cool and it starts to warm up faster than it should. 13) While there is still a little ice in the beaker, record the temperature of the solution on the data sheet. 14) Remove the beaker and water from the stir plate and use the forceps to remove the magnetic stir bar. Wipe the outside of the beaker to remove any water and re-weigh the beaker with the solution in it. Use the initial and final weights of the beaker to determine the weight of the solution. Assume the solution has a density of 1 g/ml, and determine the volume of the solution. 15) Perform calculations on the worksheet provided to determine the expected freezing point depression. Then calculate the accuracy of your observed freezing point depression. 16) Dispose of the salt solution in the Hazmat container. Clean and dry the beaker, magnet, forceps, glass rod, and thermometer in preparation for Part B.

3 Part B - Determination of the molar mass of an unknown salt The second part of the experiment is to use the colligative properties of solutions to estimate the molar mass of an unknown salt. The experimental procedure is identical to the previous procedure in terms of data collection. You will determine the freezing point depression caused by 25 grams of an unknown salt. The freezing point depression will give you (after a few calculations) the molarity of the unknown salt ions in solution, and hence the molarity of the salt. The molarity of the salt combined with the volume of the solution provide the number of moles of the salt in solution. Using the initial weight of the salt sample combined with the calculated number of moles of the unknown salt in solution, you can calculate the molar mass of the unknown salt. Experimental Procedure Part B 1) Repeat the procedure from Part A using 25 g of unknown salt in place of the NaCl. 2) Dispose of the solution in the hazard waste jug. 3) Clean all your equipment and leave your station as you found it.

4 Report Sheet Part A Freezing point Depression from Sodium Chloride Name Initial weight of empty 250 ml beaker: Weight of sodium chloride salt (~ 11.6 g): = mol NaCl Temperature of water with ice: C Final temperature of water with ice and salt: C (with ~50 ml slush left) Freezing point depression due to salt: C Final weight of beaker + water + salt: (don t forget to remove stir bar) Net weight of solution: Approximate volume of solution L (use a density of 1 g/ml) Molarity of NaCl solution (mol NaCl/volume sol n) mol/l Molarity of dissociated ions in solution mol/l The equation for the freezing point depression of water is: T = K f M Where K f is the freezing point depression constant for water (use K f = 1.7 C/M for this experiment due to the high molarities of the salts) and M is the molarity of the solute. Note that two moles of solute ions are generated from one mole NaCl, so you must use the molarity of the dissociated ions. NaCl Na + + Cl - Using the above equation, calculate the expected freezing point depression for your experiment. Expected freezing point depression = C Calculate your accuracy [(observed expected)/expected] 100 = % of 7 pts

5 Part B Molar Mass Determination of an Unknown Salt Initial weight of empty 250 ml beaker: Weight of unknown salt (you want ~ 25 g): Temperature of water with ice: C Final temperature of water with ice and salt: C (with ~50 ml slush left) Freezing point depression due to salt: C Using T = K f M, calculate the molarity of the unknown salt ions (where K f = 1.7 C/M). Your salt dissociates to form 2 moles of ions for every mole of solid salt. Molarity of salt ions (dissociated): mol/l Molarity of salt (undissociated): mol/l Final weight of beaker + water + salt: (don t forget to remove stir bar) Net weight of solution: Approximate volume of solution L (use a density of 1.0 g/ml) Using the molarity of the undissociated salt calculated above, calculate the moles of salt in your sample: Moles salt in sample: mol The molar mass of the unknown salt is the weight of the salt sample divided by the moles in the salt sample. Molar mass of unknown salt /mol of 8 pts

6 Post Lab Questions 1. What are the major sources of error in this experiment? 2. Suppose your thermometer consistently read a temperature 1.2 o lower than the correct temperature throughout the experiment. How would this have affected the molar mass you found? 3. If the freezing point of the solution had been incorrectly read 0.3 o lower than the true freezing point, would the calculated molar mass of the solute be too high or too low? Explain your answer. 4. Arrange the following aqueous solutions in order of increasing freezing points (lowest to highest temperature): 0.10M glucose, 0.10M BaCl 2, 0.20M NaCl, and 0.20M Na 2 SO What mass of NaCl is dissolved in 150g of water in a 0.050m solution? m (molality units are moles of solute per Kg solvent) 6. A molar mass determination experiment was conducted in an identical fashion as the one you conducted. The unknown salt dissociates to form two ions. The data was: Mass of unknown salt = g Initial ice + water temperature = 0.2 C Final ice+water+salt temperature = 0.4 C The initial weight of the empty beaker = g The final weight of the beaker + water+ salt = g Assume the density of water with the salt at this temperature is 1 g/ml and K f = 1.7 C/M. What is the molar mass of the unknown salt? You must show your calculations to receive credit. of 7 pts Total for lab of 25 pts

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