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Determining the Molecular Mass by Freezing Point Depression

Determining the Molecular Mass by Freezing Point Depression. By: Justin Green. Freezing Point Depression. Freezing point depression is the decrease of the freezing temperature of a solvent after a solute has been added. www.chemprofessor.com.

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Determining the Molecular Mass by Freezing Point Depression

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  1. Determining the Molecular Mass by Freezing Point Depression By: Justin Green

  2. Freezing Point Depression • Freezing point depression is the decrease of the freezing temperature of a solvent after a solute has been added www.chemprofessor.com This shows the freezing point of GEOMELT before and after adding a solute. The solute lowers the freezing point.

  3. Freezing Point Depression • Ex. The freezing point of salt water is lower than that of pure water • http://www.princeton.edu/~pccm/outreach/scsp/water_on_earth/salt/thermoms.gif

  4. Molar Mass • The mass of one mole (6.022 x 1023 molecules) of any substance: • Is given on the Periodic Table of Elements

  5. Formulas Molar Mass can be found through the formulas following with Freezing Point Depression: M = wKf w= mass fraction of solute ΔT Kf = cyroscopic constant ΔT = change in temperature ΔTf = iKfm I = van’t Hoff value Kf = molal freezing point constant m = concentration in molality

  6. Formulas to use ΔTf = I x kf x moles of solute kg of solvent ΔTf = I x kf x m ΔTf = (g solute) (MMsolute) kg solvent MMsolute = I x kf x g solute kg of solvent x ΔTf

  7. Purpose • To understand how freezing point depression and molecular mass correlate • To be able to use new knowledge of freezing point depression in a real world problem

  8. Materials required • Medium sized test tubes • 4.0g Naphthalene • 52.0g paradichlorobenzene • Thermometer • Hot Water Bath (250 mL Beaker and 100 mL water on Hot Plate)

  9. Procedures:Determining freezing point • 1. Weigh out 4g of naphthalene • 2. Weigh out 52g of paradichlorobenzene

  10. Procedures • 3. Place test tube containing chemicals in hot water bath • 4. Place both substances in the hot water bath and allow them to completely melt and stir well http://www3.moe.edu.sg/edumall/tl/digital_resources/biology/images/hot_water_bath.jpg

  11. Procedures • 5. Remove from the hot water bath and allow it to cool while stirring gently to prevent super cooling http://cn1.kaboodle.com/hi/img/2/0/0/3e/c/AAAAAs-rj2IAAAAAAD7AHg.jpg

  12. Procedures • 6. Take the temperature (down to 0.1 degrees Celsius) of the solvent every 60 seconds until the solution is solidified http://wolfesscience.com/images/lab-thermometer.jpg

  13. Freezing Point Graph I drew this one myself 

  14. Analysis Questions 1. What is the freezing point of the final solution? 48.4 °C 2. What was the freezing point depression of the solution? The standard freezing point for paradichlorobenzene is 53.0 degrees Celsius ΔTf= T°f – Tf = 53.0°C – 48.4°C = 4.6°C

  15. Analysis Questions 3. What was the molality of the naphthalene? The freezing point depression constant for paradichlorobenzene is 7.1°C x m-1 m = ΔTf = 4.6°C = 0.65m kf 7.1°C x m-1

  16. Analysis Questions 4. What was the molecular mass of naphthalene? MMsolute= kf x g solute = 7.1°C x kg kg solvent x ΔTf mol x .052 kg x 4.6°C 120 g mol-1

  17. Analysis Questions 5. What was the percent error of the results compared to the actual molecular mass of 128 g x mol-1 120 – 128 x 100% = -6% 128 The results were 6% below expected

  18. Conclusion • In conclusion, the molecular mass of an unknown solute can be determined through freezing point depression

  19. Freezing Point Depression By: Justin Green

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