Presentation on theme: "Colligative Properties: Freezing Point Depression"— Presentation transcript:
1 Colligative Properties: Freezing Point Depression General Chemistry 101/102 Laboratory Manual University of North Carolina Wilmington
2 Colligative Properties: Freezing Point Depression PurposeTo learn how to prepare Molar solutions.To study the colligative property of freezing point depression.To compare the freezing points of several salt solutions by evaluating the effect of concentration and the van’t Hoff factor (i).Safety ConsiderationsAvoid skin contact with dry ice. It will burn your skin because it is at -75 oC.Avoid skin contact with ethanol. The ethanol is located in the fume hood and can be recycled after used for the dry ice bath.
3 0.500 M x 58.5 g/mole x 0.0500 L DI water = 1.46 g NaCl Colligative Properties: FP DepressionProcedureThis experiment is performed in groups of 4. Each pair of students will be assigned 2 salt solutions to prepare and evaluate.You and your partner will prepare molar solutions of the compounds assigned to you using volumetric glassware. The following formula will help you calculate the mass of compound to add to 50 mL of DI water in a volumetric flask.Calculation of mass of NaCL required to make a M NaCl solution:0.500 M x g/mole x L DI water = 1.46 g NaCl
4 Colligative Properties: Freezing Point Dep Add the mass of your solute to a beaker containing approximately 20 mL of DI water. Swirl to dissolve the solute.Transfer this solution by pouring it into a clean volumetric flask using a funnel.Using your squeeze bottle filled with DI water, add DI water up so that the meniscus lies on the calibration mark on the neck of the flask.Cap the flask and shake the solution.
5 Colligative Properties: Freezing Point Dep. ProcedureConnect the Tablet computer, Microlab unit, and the temperature probe as shown in the illustration below.
6 Colligative Properties: Freezing Point Dep ProcedureTo open the Intermolecular Forces experiment on the computer, click on the Applications folder, and select the Microlab prompt. Then double click on the “Time and Temperature” prompt as shown below.
7 Colligative Properties: Freezing Point Dep ProcedureClick on the experiment entitled “Freezing Point Depression”, as shown below.
8 Colligative Properties: Freezing Point Dep Add ethanol to a thermos until it is approximately ¾ full.Using crucible tongs, carefully add a couple of small pellets of dry ice to the ethanol in the thermos. Do not touch the dry ice with unprotected hands. Pot holders are available to hold the thermos.Measure the temperature of the dry ice bath using the temperature probe. The optimal temperature for this experiment is -20 oC.
9 Colligative Properties: Freezing Point Dep Click the “Start” prompt to begin data collection.Add some of your prepared solution to fill a test tube 1/3 full. Stir the solution using the temperature probe and monitor the temperature closely until the temperature levels off for a couple of seconds.You will observe the solution starting to solidify and it will become hard to stir the solution.
10 Colligative Properties: Freezing Point Dep CalculationsStop data collection when the temperature levels off and solution is beginning to freeze.This value is Tf of the solution.Calculate ΔTf by subtracting Ti (the freezing point of DI water) from Tf of your solution.Tf