Colligative Properties: Freezing Point Depression and Molar Mass

Transcription

1 Colliative Properties: Freezin Point Depression and Molar Mass EXPERIMENT 17 Prepared by Edward L. Brown, Lee University The student will become familiar with properties inherent to solutions and will use them to determine the molar mass of a solute. A P P A R A T U S 20 cm test tube (2) Manetic stir bar (or looped wire to stir the solution) Beaker 600 ml Stir Plate Erlenmeyer Flask (250 ml) Glass stirrin rod (1) Thermometer (-10 ºC 110 ºC) OBJECTIVE APPARATUS AND CHEMICALS C H E M I C A L S Cyclohexane Unknown 1 and Unknown 2 NaCl (salt) Freezin Point Depression of Cyclohexane 1. Tare an Erlenmeyer flask on the balance and place a clean, dry test tube (~20 cm) / stirrin bar in the flask. Weih to the nearest [Data Sheet Q1]. 2. Add ~15 ml cyclohexane. Weih the test tube / cyclohexane to the nearest [Data Sheet Q2]. 3. Determine the mass of cyclohexane [Online Report Sheet Q3] 4. Fill a 600-mL beaker half full of ice and cover the top layer of ice with table salt (NaCl). 5. Add water slowly until it is ~1 inch below the top of the ice. Crush / stir the salt/ice water ently with a stirrin rod. PROCEDURE Copyriht 2005 Chem21 LLC. No part of this work may be reproduced, transcribed, or used in any form by any means raphic, electronic, or mechanical, includin, but not limited to, photocopyin, recordin, tapin, Web distribution, or information storae or retrieval systems without the prior written permission of the publisher. For permission to use material from this work, contact us at info@chemistrylabmanual.com. Printed in United States of America.

2 Colliative Properties: Freezin Point Depression and Molar Mass 6. Make sure the thermometer is clean and dry. Insert it into the test tube containin the cyclohexane from Step 2. Do Not Remove the Thermometer From the Test Tube Durin the Experiment!! 7. Add a small manetic stir bar (or fashion a wire with a loop at the bottom that fits over the thermometer and is moved up and down to stir the solution). 8. Allow the test tube to cool in the ice/salt/water bath until obvious freezin has occurred (~25% IS FROZEN; 75% IS STILL LIQUID). 9. Place a stir plate on your bench next to a rin stand. 10. Place the test tube in an Erlenmeyer flask on the stir plate and adjust the speed of the motor to stir as fast as possible. Make sure your thermometer is not touchin the spinnin stir bar by placin a paper towel snuly in the mouth of the test tube. 11. Allow the liquid to warm and record the temperature (to the nearest 0.2ºC) when about twenty crystals are left swirlin around in the cold liquid the thermometer should be left in the cold solution while the temperature is read. 12. Repeat Steps 8-11 until you arrive at a consistent freezin point for cyclohexane (two of your readins are within 0.2 ºC of one another). 13. Record the freezin point of pure cyclohexane to the nearest 0.2 ºC [Data Sheet Q4]. 14. SAVE THIS CYCLOHEXANE FOR THE NEXT PROCEDURE make sure you leave the thermometer in the liquid so as to minimize the loss of cyclohexane. Procedure to Determine the Molar Mass of Unknown 1: Thermometer (Your locker) Paper Towel to hold Thermometer Lare Test Tube (Lare Cabinet) Erlenmeyer Flask Manetic Stir Bar (From Instructor) Stirrin Hot Plate (Shelves at Entrance to Lab) 15. Use weihin paper to weih ± 0.05 of Unknown 1 to the nearest Record the mass of Unknown 1 [Data Sheet Q5] 16. Clean up any spills near the balance. 17. Dissolve the unknown in the cyclohexane from Step 14. Warm water can be used to help dissolve the solid. THE SOLID MUST BE COMPLETELY DISSOLVED!! 18. Place the test tube in the ice/salt water bath until obvious freezin has occurred (if freezin doesn t occur in 5 minutes, add more salt and/or ice to your bath). 19. Place the test tube in an Erlenmeyer flask on the stir plate and adjust the speed of the motor to stir as fast as possible. Make sure your thermometer is not touchin the spinnin stir bar by placin a paper towel snuly in the mouth of the test tube. 20. Allow the liquid to warm and record the temperature (to the nearest 0.2ºC) when about ten crystals are left swirlin around in the cold liquid the thermometer should be left in the cold solution when the temperature is read. 21. Repeat Steps until you arrive at a consistent freezin point for cyclohexane / Unknown 1 (two of your readins must be within 0.2 ºC of one another). 22. Record this freezin point to the nearest 0.2 ºC [Data Sheet Q6]. Experiment

3 Colliative Properties: Freezin Point Depression and Molar Mass 23. Dispose of this cyclohexane / Unknown 1 solution in the waste container in the hood. Do Not Allow The Manetic Stir Bar To Fall Into This Container!! 24. Calculate the molar mass of Unknown 1 usin Equation 17.9 [Online Report Sheet Q7]. Procedure to Determine the Molar Mass of Unknown 2: 25. Weih a clean, dry test tube (~20 cm) and stirrin bar to the nearest [Data Sheet Q8]. 26. Add between 10 and 12 of cyclohexane. Weih the test tube / cyclohexane to the nearest [Data Sheet Q9]. 27. Use weihin paper to weih ± 0.05 of Unknown 2 to the nearest Record the mass of Unknown 2 [Data Sheet Q10] 28. Clean up any spills near the balance. 29. Dissolve the unknown in the cyclohexane. Warm water can be used to help dissolve the solid. THE SOLID MUST BE COMPLETELY DISSOLVED!! 30. Place the test tube in the ice/salt water bath until obvious freezin has occurred (if freezin doesn t occur in 5 minutes, add more salt and/or ice to your bath). 31. Place the test tube in an Erlenmeyer flask on the stir plate and adjust the speed of the motor to stir as fast as possible. Make sure your thermometer is not touchin the spinnin stir bar by placin a paper towel snuly in the mouth of the test tube. 32. Allow the liquid to warm and record the temperature (to the nearest 0.2ºC) when about ten crystals are left swirlin around in the cold liquid the thermometer should be left in the cold solution when the temperature is read. 33. Repeat Steps until you arrive at a consistent freezin point for cyclohexane / Unknown 2 (two of your readins are within 0.2 ºC of one another). 34. Record this freezin point to the nearest 0.2 ºC [Data Sheet Q11]. 35. Dispose of this cyclohexane / Unknown 2 solution in the waste container in the hood. Do Not Allow The Manetic Stir Bar To Fall Into This Container!! 36. Calculate the molar mass of Unknown 2 usin Equation 17.9 [Online Report Sheet Q12]. For Cyclohexane, ΔT = C molality Since, the chane in temperature is represented by: Δ T = T T cyclohexane with unknown pure cyclohexane and the molality is molality (m) = moles solute k solvent and another way to express moles of solute is rams solute moles solute = Molar Mass solute Experiment

4 Colliative Properties: Freezin Point Depression and Molar Mass One can now use Equation 17.9 to determine the molar mass of the unknown solutes. k f of Cyclohexane from Table 17.1 The mass of Unknown 1 or 2 Tcyclohexane + unknown -T pure cyclohexane = 20.4 m rams solute Molar Mass solute k solvent Equation 17.9 The Two Temperature Measurements Determined from the mass of cyclohexane Waste Disposal Dispose of all cyclohexane solutions in the waste container in the hood. Return The Manetic Stir Bar To Your Instructor. Lab Report: Once you have turned in your Instructor Data Sheet, lab attendance will be entered and lab attendees will be permitted to access the online data / calculation submission part of the lab report (click on Lab 17 Colliative Properties). Enter your data accurately to avoid penalty. The lab proram will take you in order to each calculation. If there is an error, you will be iven additional submissions (the number and penalty to be determined by your instructor) to correct your calculation. Post-Lab Questions: The questions for this lab can be found at Do Not Wait Until The Last Minute!!!! Computer Problems and Internet Unavailability Happen, But Deadlines Will Not Be Extended!! On the Internet, complete the Post Lab Questions at the end of Lab 17 Colliative Properties. The computer proram will check your answers (with the correct number of sinificant fiures) as they are entered. If there is an error, you will be iven additional submissions (the number and penalty to be determined by your instructor) to correct your answer. Late Submission: Late submission of the lab data / calculations is permitted with the followin penalties: - 10 points for submissions up to 1 day late, - 20 points for submissions up to 2 days late. Experiment

5 Laboratory 17 Student Data Sheet 1 Mass of test tube + stirrin bar 2 Mass of test tube + stirrin bar + cyclohexane 4 Freezin point of pure cyclohexane 5 Mass of Unknown 1 6 Freezin point of cyclohexane + Unknown 1 8 Mass of test tube + stirrin bar 9 Mass of test tube + stirrin bar + cyclohexane 10 Mass of Unknown 2 11 Freezin point of cyclohexane + Unknown 2 Laboratory 17 Instructor Data Sheet Name: 1 Mass of test tube + stirrin bar 2 Mass of test tube + stirrin bar + cyclohexane 4 Freezin point of pure cyclohexane 5 Mass of Unknown 1 6 Freezin point of cyclohexane + Unknown 1 8 Mass of test tube + stirrin bar 9 Mass of test tube + stirrin bar + cyclohexane 10 Mass of Unknown 2 11 Freezin point of cyclohexane + Unknown 2 Experiment