Spectroscopy: Is There Life Out There (2024)

TITLEOF LESSON: Colligative Properties

LENGTHOF LESSON: 50 minutes

NAMEOF COURSE: Chemistry

GRADELEVEL: High School

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SOURCEOF THIS LESSON:

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Chemweb.Colligative Properties of Solutions: Freezing Point Depression. Retrieved 3 Oct, 2006.

<http://chemweb.calpoly.edu/chem/125/125LabExp/FPDepression/Doing>

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TEKSADDRESSED:

Environmental Sciences:

(13)Science concepts. The student knowsrelationships among the concentration, electrical conductivity, and colligativeproperties of a solution. The student is expected to:

(B)interpret relationships among ionic andcovalent compounds, electrical conductivity, and colligative properties ofwater; and

Chemistry

(12)Science concepts. The student knows the factorsthat influence the solubility of solutes in a solvent. The student is expectedto:

(A)demonstrate andexplain effects of temperature and the nature of solid solutes on thesolubility of solids;

(B)develop generalrules for solubility through investigations with aqueous solutions; and

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PERFORMANCEOR LEARNER OUTCOMES:

Studentswill be able to:

<![if !supportLists]>¤<![endif]>Define colligativeproperties and give four examples of colligative properties

<![if !supportLists]>¤<![endif]>Use experimental data toestimate vanÕt HoffÕs factor

<![if !supportLists]>¤<![endif]>Use the Freezing PointDepression equation to calculate the Molarity of an unknown solutioin

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RESOURCES,MATERIAL AND SUPPLIES NEEDED:

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• ice
• rock salt
• various solutions
• unknown solution
• large test tubes (17x100mm ones in the middle of the room)
• temperature probe attached to Vernier interface
• 600-mL beaker
• 50 mL graduated cylinder

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SAFETYCONSIDERATIONS:

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Be careful with thethermometers; they are delicate and can break. These solutions will become verycold and should not be left in contact with studentsÕ skin for any amount oftime.

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SUPPLEMENTARY MATERIALS,HANDOUTS:

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<![if !supportLists]>¤<![endif]>Lab Guide

<![if !supportLists]>¤<![endif]>Calculation Sheet

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LESSON PLAN

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Name_________________________

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Colligative Properties

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-----<![if !vml]><![endif]>MATERIALS

Chemicals:

• ice
• rock salt
• various solutions
• unknown solution

-----<![if !vml]><![endif]>EQUIPMENT

• large test tubes (17x100mm ones in the middle of the room)
• temperature probe attached to Vernier interface
• 600-mL beaker
• 50 mL graduated cylinder

-----<![if !vml]><![endif]>SAFETY PRECAUTIONS:

Goggles must be worn throughout the experiment.

-----<![if !vml]><![endif]>:BACKGROUND:

The purpose of this experiment is todemonstrate the effect of different solutes on the freezing point of water. Inthis experiment, we'll use the colligative property freezing pointdepression (ΔT_fp) todetermine the van't Hoff factor i for different known solutes and the molar mass of an unknown solute.

In the first part of the experiment, varioussolutes are added to water and the resultant freezing points of the solutionsare determined. The actual value of the van't Hoff factor i, or number of dissolved particles per formula unit,is determined from the measured freezing point. This is then compared to theideal van't Hoff factor. In the second part, the molar mass of an unknownsolution is determined. Calculations involve use of the following equation forthe change in temperature (in¡C) :

ΔT_fp = K_fm ¥ i

K_f is a constantfor the solvent in ¡C/ m (weÕll name this constant later)

m is the molality of the solution in mol solute/kgsolvent

i is the van't Hoff factor

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-----<![if !vml]><![endif]>PROCEDURE:

As usual, all data should berecorded in your lab notebooks. Eachpair at your tablewill investigate one of four known solutions. You will share your data withothers at your table so that each pair has data on four solutions.

NOTE: The amount of solute per gramof solvent will be indicated on the stock bottle of each solution, you willrecord this information and use it to calculate the molality of the solution.

Preparation of Ice Bath

1. Fill the beaker 3/4 full with ice.
2. Cover the ice with 1/4 to 1/2 inches of rock salt.
3. Stir this ice-salt mixture with a stirring rod and make sure the temperature drops to -5¡C or below.

Determination of FreezingPoint of Pure Water

1. Make sure your temperature probeis connected to the interface. Open LoggerPro using the FP.xmbl file found onthe desktop in the Chem 125 folder. Make sure you always use this file and not"untitled".

2. Place a test tube that is 1/2full of deionized water in the ice bath.

3. Place the temperature probe inthe middle of the water layer. Then press "collect" and stir thewater in the test tube gently with the temperature probe while LoggerProrecords the temperature.

IMPORTANT: Keep theprobe off of the bottom of the test tube. Why do you think you need to do this?

4. When the first ice crystalsappear on the inside wall of the test tube, you should observe the recordedtemperature to level off. This should be the freezing point of pure water.Record the value in your lab notebook to the proper number of significantfigures.

Determination of FreezingPoints of Solutions:

1. Obtain about 25 mL of yourassigned solution in your graduated cylinder. Make sure you record the identityof the solution as well as the given concentration.

2. Repeat steps 2-4 with thissolution to record the freezing point of the solution. You should do this atleast three times. Take the average of the measured freezing point data.

3. Share your freezing point datafor your solution with the others at your table*. Get freezing point data forthree other solutions from the other students at your table. Plotrepresentative freezing curves for each solution on the same graph, making sureto clearly distinguish each curve. Turn in this graph with your report.

4. Calculate the molalities of eachsolution from the given concentration information.

5. Using the measured freezing pointdata and your calculated molalities, determine the value of i for each solution. K_f for water = 1.86¡C/m.

* You will want to share data aftertransferring it to Excel. Probably the easiest way to do this is to firsttransfer your data (copy & paste) from LoggerPro to Excel, give the Excelworkbook a distinct name that includes your computer number, then save yourExcel workbook and quit Excel. You can email the file to others.

Molar Mass Determination FromFreezing Point Data

1. Obtain an unknown solution, makesure you record the unknown number as well as given concentration informationin your notebook. Unless instructed otherwise,each partner should have their own unknown!

2. Freeze this solution in the samemanner as you did the previous solutions. Be sure to record the freezing pointtemperature.

3. Calculate the molecular mass ofthis solute based on the freezing point depression (i = 1.0).

WASTE DISPOSAL

Dispose of the solutions asinstructed.

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