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Chemistry

Name:__________________

Period: ____

Colligative Properties and Types of Solutions

Packet 12

Day In Class Work 1 Molality

2 Freezing Point Depression

3 Freezing Point Depression Lab

4 Boiling Point Elevation

5 Test

6 Ice Cream Lab

Outcomes 1. Solve freezing point depression problems

2. Solve boiling point elevation problems

3. Write an essay on the ice cream experiment explaining the principles of freezing point

depression and types of colloids.

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In Packet 11 we will investigate colligative properties and colloidal properties of matter.

Colligative properties are properties of solutions that depend on the number of particles in a

given volume of solvent and not on the mass of the particles. Colligative properties include:

lowering of vapor pressure; elevation of boiling point; depression of freezing point; osmotic

pressure.

MOLALITY The molality of a solution is calculated by taking the moles of solute and dividing by the

kilograms of solvent.

Example - What is the molality when 0.75 mol is dissolved in 2.50 kg of solvent?

The answer is 0.300 m.

Complete the following questions and problems relating to molality.

1) Write the equation for molality:

2) Write the equation for molarity:

3) Explain in words how molality and molarity differ.

4) What is the molality of a solution in which 0.32 moles AlCl3 has been dissolved in 2,200 g

water?

5) What is the molality of a solution in which 0.60 mol propyleneglycol are dissolved in 2kg of

water??

6) What is the molality of a solution in which 0.145 mol CO2 (molar mass = 44.01

g/mol) is dissolved in 591 g water?

7) What is the molality of a solution in which 13.7 g NaCl has been dissolved in

500.0g water?

8) How many grams of ethanol, C2H6O (molar mass 46.08 g/mol), are need to prepare a 0.10

molal solution using 1.000 kg water.

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Molality Worksheet

1. What is the molality of a solution that contains 63.0 g HNO3 in 0.500 kg H2O?

2. What is the molality of a solution that contains .500 mol HC2H3O2 in 0.125 kg H2O?

3. How many grams of water is required to dissolve 100. g NaCl to prepare a 1.50 m

solution?

4. How many grams of water must be used to dissolve 0.500 kg C2H5OH to prepare a 3.00

m solution?

5. How many grams of H2SO4 must be dissolved to 2.40 kg H2O to produce a 1.20 m

solution?

6. What is the number of molecules of C2H5OH in a 3 m solution that contains 4.00 kg

H2O?

7. What is the molality of a solution that contains 80.0 g Al2(SO4)3 in 625 g H2O?

8. How many grams of water is required to dissolve 175 g KNO3 to produce a 2.25 m

solution?

9. What mass of HC2H3O2 must be dissolved in 800. g H2O to produce a 6.25 m solution?

10. How many moles of NH4+ ions are dissolved in 0.750 kg of H2O when the concentration

of (NH4)3PO4 is 0.400 m?

Concentration Worksheet

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1) If I make a solution by adding 83 grams of sodium hydroxide to 750 mL of water…

a) What is the molality of sodium hydroxide in this solution?

b) What is the percent by mass of sodium hydroxide in this solution?

c) What is the mole fraction of sodium hydroxide in this solution?

2) If I make a solution by adding water to 35 mL of methanol (CH3OH) until the final

volume of the solution is 275 mL…

a) What is the molarity of methanol in this solution? (The density of

methanol is 0.792 g/mL)

b) What is the percent by volume of methanol in this solution?

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Freezing-point depression describes the phenomenon that the freezing point of a

liquid (a solvent) is depressed when another compound is added, meaning that a solution has

a lower freezing point than a pure solvent. This happens whenever a solute is added to a pure

solvent, such as water. The phenomenon may be observed in sea water, which due to its salt

content remains liquid at temperatures below 0°C, the freezing point of pure water.

The freezing point depression ΔTf is a colligative property of the solution.

ΔTfp = Kfpm

ΔTfp = freezing point depression Kfp = molal freezing constant m = molality

Solvent Regular Freezing

Point

(°C)

Kfp

(°C / m)

Acetic acid 16.60 3.90

Benzene 5.5 4.90

Camphor 179.8 37.7

Naphthalene 80.2 6.9

Phenol 40.9 7.4

Water 0.0 1.86

Actual Freezing Point = Regular Freezing Point - ΔTfp

Solve the following problems. Show all work.

1. Calculate the freezing point depression of a 0.25m solution of NaCl in benzene.

2. Calculate the freezing point depression for a 0.15m solution of KCl in water.

3. Calculate the actual freezing point of a benzene solution if the freezing point depression

of the solution is 1.275°C.

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4. Calculate the actual freezing point of a 1.5m NaI solution in water.

5. What would be the actual freezing point of a solution made by dissolving 20.5 grams of

I2 in 100 grams of naphthalene?

6. A benzene solution is found to have an actual freezing point of 4.2°C. What is the

molality of the solution?

7. Calculate the freezing point depression of a 2.5m solution of KCl in acetic acid.

8. A solution of I2 in camphor is found to have an actual freezing point of 173.5°C. What

is the molality of the solution?

9. What would be the actual freezing point of a solution made by dissolving 3 grams of

NaCl in 25 grams of water?

MOLECULAR WEIGHT BY FREEZING POINT DEPRESSION Purpose: The purpose of this laboratory activity is to determine the molecular weight of saline solution

using the technique of freezing point depression.

Procedure:

Part I: Freezing Point of Water

1. Obtain a clean, dry, large test tube. Place about 10 mL of distilled water in the test tube.

Determine the mass of the water used. Record the mass of the water in the data table.

2. Prepare an ice bath in a foam cup with ice and rock salt. Place the cup in a 250mL beaker to

give it more stability. The ice bath should be deep enough so that it is above the level of the

water in the test tube and well below the top. Take care not to let any of the salt or ice get into

the sample of distilled water.

3. Place a thermometer in the distilled water. Take time-temperature data every half-minute

until ice has formed in the test tube. It is not necessary to freeze the entire sample. Record the

temperature at which the sample froze.

4. Do not discard the sample of the distilled water, because the sample will be used in Part II.

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Part II: Molecular Weight of the Unknown

5. Remove the test tube containing the distilled water from the ice bath. Allow the ice to melt.

This step can be speeded up by placing the test tube or vial in a beaker of tap water.

6. Weigh out approximately 1 gram of NaCl. Record the mass of the sample in the data table.

Add the NaCl to the distilled water, and stir until it is all dissolved. Return the test tube to the

ice bath. Insert the thermometer.

7. Take time-temperature data as in Part I. Again, the sample does not have to be frozen solid in

order to determine the freezing point. Record the freezing point in the data table.

8. Repeat the procedure (both Parts I and II) with sugar.

Time-Temperature Data:

Pure Water Salt trail I Pure Water Sugar trial II

Data Table:

Trial I NaCl Trail II C6H12O6

Mass of water Mass of water

Tf water Tf water

Mass of salt Mass of sugar

Tf solution Tf solution

∆ T ∆ T

molality molality

Trial 1 Part I Trial 1 Part II Trial II Part I Trial II Part II

Time Temp Time Temp Time Temp Time Temp

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Calculations:

1. Calculate the experimental freezing point depression of the salt and sugar solution.

2. Calculate the accepted freezing point depression of the salt and sugar solutions.

3. Determine the percent error for each solution.

4. List 3 sources of error.

Questions: 1. What differences is expected of a covalent compound such as sugar and an ionic bond like

salt?

2. Why is it not necessary to wait for the entire sample of water to freeze in order to determine its

freezing point?

3. Why is it a good idea to measure the freezing point of the water instead of assuming that its

freezing point is exactly 0o C?

4. What would have happened if a two-gram sample of salt were used in this experiment?

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Boiling-point elevation describes the phenomenon that the boiling point of a liquid (a

solvent) will be higher when another compound is added, meaning that a solution has a higher

boiling point than a pure solvent. This happens whenever a non-volatile solute, such as a salt, is

added to a pure solvent, such as water.

The boiling point elevation ΔTbp is a colligative property of the solution.

ΔTbp = Kbpm

ΔTbp = boiling point elevation Kbp = molal freezing constant m = molality

Solvent Regular Boiling

Point

(°C)

Kbp

(°C / m)

Acetic acid 117.9 3.07

Benzene 80.15 2.53

Camphor 204.0 5.95

Naphthalene 218.0 5.65

Phenol 81.6 3.56

Water 100.0 0.51

Actual Boiling Point = Regular Boiling Point + ΔTbp

Solve the following problems. Show all work.

1. Calculate the boiling point elevation of a 0.2m solution of NaCl in water.

2. Calculate the actual boiling point of the solution in problem #1.

3. Calculate the boiling point elevation for a 0.4m solution of NaCl in phenol. Calculate

the actual boiling point of the solution.

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4. A water solution is found to have an actual boiling point of 100.85°C. What is the

molality of the solution?

5. A solution of iodine in benzene is found to have an actual boiling point of 81.9°C. What

is the molality of the solution?

6. A solution is prepared by dissolving 24.2 grams of sugar in 200grams of water. What is

the actual boiling point of the solution?

7. A benzene solution is found to have an actual boiling point of 83.9°C. What is the

molality of the solution?

Review

1. Calculate the actual freezing point of 1.25m solution of KCl in water.

2. Calculate the boiling point elevation of 0.5m solution of KCl in phenol

3. Calculate the actual freezing point of a 1.35m solution of NaCl in benzene.

4. Calculate the freezing point depression of a 2.4m solution of NaCl in water.

5. Calculate the boiling point elevation of a 1.75m solution of KCl in camphor.

6. Calculate the molality of phenol in a solution if the actually freezing point is 37.5°C.

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Boiling Point Elevation

Boiling point elevation occurs when the boiling point of a solution becomes higher than the

boiling point of a pure solvent. The temperature at which the solvent boils is increased by adding

any non-volatile solute. A common example of boiling point elevation can be observed by

adding salt to water. The boiling point of the water is increased.

Purpose: Determine the boiling point of pure water, salt water and sugar water.

Procedure:

1. Get the mass of 150ml of water and place it in a 250ml beaker.

2. Suspend a thermometer in the 250ml beaker so it sit in the middle of the water and record

the temperature.

3. Light the Bunsen burner and record the temperature of the water every 30 seconds for 8

minutes in the data table below.

4. Discard the water.

5. Measure out 2 grams of NaCl and place it 150ml of water. Place the solution in a 250ml

beaker. Repeat step 2 and 3 for the salt water solution.

6. Discard saline solution

7. Measure out 2 grams of sugar and place it in a 150ml of water. Place the sugar solution

in a 250ml beaker and repeat steps 2 and 3.

8. Discard sugar solution

Data:

Time (min) Pure Water

(°C)

Saline Solution

(°C)

Sugar Solution

(°C)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

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Mass of 150ml of water ______ Mass of salt _______ Mass of sugar ______

Kilograms of water ______ Moles of Salt ______ Moles of sugar ______

Molality of Salt Water ____________ Molality of Sugar Water __________

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Graph the information from the Data Table

Calculations:

What is the accepted boiling point of water?

What is the experimental boiling point of water?

Percent Error?

ΔTbp = Kbpm

What is the accepted boiling point of salt water?

What is the experimental boiling point of salt water?

Percent Error?

ΔTbp = Kbpm

What is the accepted boiling point of sugar water?

What is the experimental boiling point of sugar water?

Percent Error?

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Colligative Properties

PART A – CALCULATIONS

1. Indicate how many ions are formed when the following solutes dissolve.

SOLUTE # OF IONS SOLUTE # OF IONS

sucrose (C12H22O11) magnesium chloride (MgCl2)

sodium sulfate (Na2SO4) methanol (CH3OH)

2. When 5.0 g of CaCl2 dissolves in 50.0 g of water, what is the boiling point of the solution?

GIVEN WORK

3. Find the boiling point of a solution containing 6.0 g benzene, C6H6, in 35 g of napthalene.

GIVEN WORK

4. Mrs. J’s feet are aching at the end of a long day. At home, she dissolves 26.0 g of Epsom salt, MgSO4, in 1.5 kg of water. What is the freezing point of this solution?

GIVEN WORK

PART B – APPLICATIONS (Answer the following questions on the back of this paper.)

5. Salt is often used to remove ice from roads and sidewalks. Explain how this process works in terms of colligative properties.

6. Which salt, NaCl or CaCl2, has a greater effect on freezing point? Explain.

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The CHEMISTRY of Ice Cream

Who knew chemistry could be so tasty! Today we will be investigating colligative properties

and how they affect freezing points, and some yummy results of energy changes.

Pre-Lab questions:

1. What is freezing point depression?

2. Is freezing an exothermic or endothermic process?

3. Is melting an exothermic or endothermic process?

4. How does energy flow? (from where to where?)

OBJECTIVES:

Investigate the effects of temperature change on phase changes

Investigate the effects of changes in freezing point

Utilize the law of conservation of energy

Apply these concepts to make ice cream!

RECIPE:

Ingredients:

Write your final recipe here. It must be approved before you begin.

Analysis:

1. What state of matter was the milk when you began?

2. What state of matter was the milk when you were done?

3. In order to change the phase of the milk, what had to be removed?

4. What happened to the heat energy that left the milk?

5. Why was salt added to the ice?

6. If you did not add sugar, would the ice cream freeze faster or slower? Why?

Explain, using your knowledge of chemistry.

7. Describe the transfer of energy that occurred in this lab.

8. How could you improve your recipe?

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Colligative Properties Worksheet

Complete the following questions and problems relating to colligative properties.

1) Write the mathematical expression for boiling point elevation. Define all of the terms used.

2) Write the mathematical expression for freezing point depression. Define all of the terms

used.

3) When sugar is dissolved in water, will the resulting solution boil at a higher or lower

temperature than pure water? Explain.

4) When salt is dissolved in water, will the resulting solution freeze at a higher or lower

temperature than pure water? Explain.

5) What is the change in boiling point (vs. pure water) of a 1.25 molal solution of sugar in

water? The Kb for water is 0.52oC/m.

6) What is the change in freezing point (vs. pure water) of a 0.78 molal solution of NaCl in

water? The Kf for water is 1.86oC/m.

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7) A solution was prepared by dissolving 0.42 mol hexane into 500g CCl4. What is the change

in freezing point (vs. pure CCl4) of this solution? The Kf for CCl4 is 30.0oC/m.

8) A solution was prepared by dissolving 0.16 mol ethanol into 750g ether. What is the change

in boiling point (vs. pure ether) of this solution? The Kb for ether is 2.02oC/m.

9) A solution of propylene glycol (antifreeze) in your car’s radiator has a freezing point of –

20oC. What is the molality of the solution? Kf for water is 1.86

oC/m.

10) In order to make spaghetti cook faster, a chef adds salt to water. How many moles of salt

would he need to add to 1.0 kg water to make the water boil at 105oC? The Kb for water is

0.51oC/m.

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Colligative Properties Worksheet

1. For a 0.222 m aqueous solution of sucrose, C12H22O11, What is the boiling point?

2. What is the normal boiling point of a 0.122 m aqueous solution of KI?

3. Automotive antifreeze consists of ethylene glycol, C2H6O2 , a nonvolatile nonelectrolyte.

Calculate the boiling point and freezing point of a 25.0 mass percent solution of ethylene

glycol in water.

4. Calculate the freezing point of a solution containing 0.600 kg of CHCl3 and 42.0 g of

eucalyptol9 C10H18O), a fragrant substance found in the leaves of eucalyptus tree. Kf=

4.68 0c/m, normal freezing point = -63.5

0c

5. List the following aqueous solutions in order of their expected freezing points: 0.050 m

CaCl2; 0.15 m NaCl; 0.10 m HCl; 0.050 m HC2H3O2; 0.10 m C12H22O11

6. A solution of an unknown nonvolatile nonelectrolyte was prepared by dissolving 0.250 g

in 40.0 g CCl4. The normal boiling point of the resultant solution increased by 0.3570C.

Calculate the molar mass of the solute. Kb= 5.02 0c/m

7. Camphor, C10H16O, melts at 179.8 o C, it has particularly large freezing point depression

constant, Kf = 400C/mol. When 0.186 g of an organic substance of unknown molar mass

is dissolved in 22.01 g of liquid camphor, the freezing point of the mixture is found to be

176.7 0C. What is the approximate molar mass of the solute?

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