Chemistry Name: Period: Colligative Properties and Types of Solutions Packet 12

Transcription

1 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. 1

2 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 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 AlCl 3 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 mol CO 2 (molar mass = 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, C 2 H 6 O (molar mass g/mol), are need to prepare a 0.10 molal solution using kg water. 2

3 Molality Worksheet 1. What is the molality of a solution that contains 63.0 g HNO 3 in kg H 2 O? 2. What is the molality of a solution that contains.500 mol HC 2 H 3 O 2 in kg H 2 O? 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 kg C 2 H 5 OH to prepare a 3.00 m solution? 5. How many grams of H 2 SO 4 must be dissolved to 2.40 kg H 2 O to produce a 1.20 m solution? 6. What is the number of molecules of C 2 H 5 OH in a 3 m solution that contains 4.00 kg H 2 O? 7. What is the molality of a solution that contains 80.0 g Al 2 (SO 4 ) 3 in 625 g H 2 O? 8. How many grams of water is required to dissolve 175 g KNO 3 to produce a 2.25 m solution? 9. What mass of HC 2 H 3 O 2 must be dissolved in 800. g H 2 O to produce a 6.25 m solution? 10. How many moles of NH 4 + ions are dissolved in kg of H 2 O when the concentration of (NH 4 ) 3 PO 4 is m? Concentration Worksheet 3

4 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 (CH 3 OH) 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 g/ml) b) What is the percent by volume of methanol in this solution? 4

5 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 ΔT f is a colligative property of the solution. ΔT fp = K fp m ΔT fp = freezing point depression K fp = molal freezing constant m = molality Solvent Regular Freezing Point ( C) K fp ( C / m) Acetic acid Benzene Camphor Naphthalene Phenol Water Actual Freezing Point = Regular Freezing Point - ΔT fp 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 C. 5

6 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 I 2 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 I 2 in camphor is found to have an actual freezing point of 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. 6

7 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 Trial 1 Part I Trial 1 Part II Trial II Part I Trial II Part II Time Temp Time Temp Time Temp Time Temp Data Table: Mass of water T f water Mass of salt T f solution T molality Trial I NaCl Trail II C6H12O6 Mass of water T f water Mass of sugar T f solution T molality 7

8 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 0 o C? 4. What would have happened if a two-gram sample of salt were used in this experiment? 8

9 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 ΔT bp is a colligative property of the solution. ΔT bp = K bp m ΔT bp = boiling point elevation K bp = molal freezing constant m = molality Solvent Regular Boiling Point ( C) K bp ( C / m) Acetic acid Benzene Camphor Naphthalene Phenol Water Actual Boiling Point = Regular Boiling Point + ΔT bp 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. 9

10 4. A water solution is found to have an actual boiling point of 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. 10

11 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 Discard sugar solution Data: Time (min) Pure Water ( C) Saline Solution ( C) Sugar Solution ( C) 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 11

12 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? ΔT bp = K bp m What is the accepted boiling point of salt water? What is the experimental boiling point of salt water? Percent Error? ΔT bp = K bp m What is the accepted boiling point of sugar water? What is the experimental boiling point of sugar water? Percent Error? 12

13 Colligative Properties PART A CALCULATIONS 1. Indicate how many ions are formed when the following solutes dissolve. SOLUTE # OF IONS SOLUTE # OF IONS sucrose (C 12 H 22 O 11 ) magnesium chloride (MgCl 2 ) sodium sulfate (Na 2 SO 4 ) methanol (CH 3 OH) 2. When 5.0 g of CaCl 2 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, C 6 H 6, 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, MgSO 4, 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 CaCl 2, has a greater effect on freezing point? Explain. 13

14 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? 14

15 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 K b for water is 0.52 o C/m. 6) What is the change in freezing point (vs. pure water) of a 0.78 molal solution of NaCl in water? The K f for water is 1.86 o C/m. 15

16 7) A solution was prepared by dissolving 0.42 mol hexane into 500g CCl 4. What is the change in freezing point (vs. pure CCl 4 ) of this solution? The K f for CCl 4 is 30.0 o C/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 K b for ether is 2.02 o C/m. 9) A solution of propylene glycol (antifreeze) in your car s radiator has a freezing point of 20 o C. What is the molality of the solution? K f for water is 1.86 o C/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 105 o C? The K b for water is 0.51 o C/m. 16

17 Colligative Properties Worksheet 1. For a m aqueous solution of sucrose, C 12 H 22 O 11, What is the boiling point? 2. What is the normal boiling point of a m aqueous solution of KI? 3. Automotive antifreeze consists of ethylene glycol, C 2 H 6 O 2, 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 kg of CHCl 3 and 42.0 g of eucalyptol9 C 10 H 18 O), a fragrant substance found in the leaves of eucalyptus tree. K f = c/m, normal freezing point = c 5. List the following aqueous solutions in order of their expected freezing points: m CaCl 2 ; 0.15 m NaCl; 0.10 m HCl; m HC 2 H 3 O 2 ; 0.10 m C 12 H 22 O A solution of an unknown nonvolatile nonelectrolyte was prepared by dissolving g in 40.0 g CCl 4. The normal boiling point of the resultant solution increased by C. Calculate the molar mass of the solute. K b = c/m 7. Camphor, C 10 H 16 O, melts at o C, it has particularly large freezing point depression constant, K f = 40 0 C/mol. When g of an organic substance of unknown molar mass is dissolved in g of liquid camphor, the freezing point of the mixture is found to be C. What is the approximate molar mass of the solute? 17

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