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# WVU CHEM 116 - 116_03_colligative_properties

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Chem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 1 of 12 Solubility of Solids in dissolved in a liquid solvent Effect of Pressure on solubility of solids in liquid: ♦ Effect of Temperature on solubility of solids in liquid: ♦ − Solubility of gases dissolved in a liquid solvent Effect of Temperature on gas in liquid: ♦ Effect of Pressure of gas in liquid: ♦Chem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 2 of 12 Concept Check To decrease the solubility of gaseous oxygen in liquid water, you would _____ the temperature and/ or _____ the pressure. a. increase; increase b. increase; decrease c. decrease; decrease d. decrease; increase e. not change; increase Henry’s Law ⇒⇒⇒⇒ Pressure and the Solubility of Gases Cgas = kH × Pgas Cgas= kH = Pgas = The Henry’s-law constant of methyl bromide (CH3Br), a gas used as a soil fumigating agent, is k=0.159 mol L−1 atm−1 at 25 ºC. What is the solubility (in mol/L) of methyl bromide in water at 25oC and a partial pressure of 125 mm Hg? 0.0262 mole/L The solubility of CO2 in water is 2.4×10−2 M at 25oC and 0.750 atm pressure. What is the value Henry’s Law Constant for CO2 in mol L−1 atm−1? 3.2×10−2 mole·L–1·atm–1 What is the value of the Henry’s Law constant if 27 g of gaseous acetylene (C2H2, 26.04 g/mole) is dissolved in 0.50 L of liquid acetone at a pressure of 2.0 atm? 1.1 mole/(L·atm) What is the solubility of acetylene in acetone at 12 atm? 13 mole/LChem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 3 of 12 Concept Check The drawing below shows a closed system at 1 atm pressure where equilibrium exists between dissolved and un-dissolved gas particles. The pressure is increased to 2 atm and equilibrium is restored. What drawing best represents this new equilibrium? Colligative Properties ♦ The FOUR Colligative properties are… 1. Vapor pressure lowering 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure Some of life’s little mysteries are explained by colligative properties: ♦ Why do we put antifreeze in our cars? − ♦ Can we obtain freshwater from seawater (desalination)? − ♦ How do we refine petroleum? − Vapor Pressure, VP ≡ the pressure exerted by gaseous molecules above a liquid Volatile substance ≡ − − more volatile = higher VP a b c d initial 1 atm 2 atm 2 atm 2 atm 2 atmChem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 4 of 12 Vapor Pressure Lowering ⇒⇒⇒⇒ Case I - Solutions with a Nonvolatile Solute (a solid dissolved in a liquid) ♦ Raoult’s law ⇒⇒⇒⇒ 0solvsolvsolnPXP ⋅= more particles ⇒ more VP lowering Psoln = Xsolv 0solvP = Concept Check Which of the following aqueous solutions will exhibit the lowest vapor pressure? Assume 100% dissolution of ionic compounds. a. 1.0 M C12H22O11 b. 1.0 M NaNO3 c. 1.0 M C3H7OH d. 1.0 M MgCl2Chem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 5 of 12 What is the vapor pressure (in mm Hg) of a solution prepared by dissolving 5.00 g of benzoic acid (C7H6O2, 122.13 g/mole) in 100.00 g of ethyl alcohol (C2H6O, 46.08 g/mole) at 35oC? The vapor pressure of pure ethyl alcohol at 35oC is 100.5 mm Hg. Psoln = 98.64 mmHg What mass of sucrose (342.3 g/mole) must be added to 320. g of water to lower the vapor pressure by 1.5 mmHg at 25 ºC? The vapor pressure of water at 25 ºC is 23.8 mmHg. 4.10×102 g Vapor Pressure Lowering ⇒⇒⇒⇒ Case II - Solutions with a volatile solute (two liquids mutually dissolved in each other)Chem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 6 of 12 Consider an aqueous solution of ethylene glycol where the mole fraction of ethylene glycol is 0.0890 The vapor pressure of pure water is 23.8 mm Hg, and that of pure ethyl glycol is 61.2 mm Hg at 25oC. Which liquid is more volitile, ethylene glycol or water? What is the mole fraction of water? Determine the vapor pressure of ethylene glycol and the the vapor pressure of water above the solution. What is the total pressure of the solution. Pethylene glycol = 5.45 mmHg Pwater = 21.7 mmHg Psoln = 27.1 mmHg ♦ Note: The mole fraction of the more volatile ethylene glycol is higher in the vapor phase than in the liquid phase. − The vapor is enriched in the more volatile liquid 201.0mmHg 27.1mmHg 5.4P PPX35H20glycol ethyleneglycol ethyleneglycol ethylene==+= mole fraction ethylene glycol in liquid phase mole fraction ethylene glycol in gaseous phase ♦ This is a general (and well exploited) result liquid Po at 25oC ethanol 44 mmHg water 23.8 mmHgChem 116 Lecture Note Outlines Set 03: Colligative Properties Spring 2015 (Ratcliff) Everything in life comes to you as a teacher. Pay attention. Learn quickly. Page 7 of 12 Consider a solution containing 25.0 grams of water (18.02 g/mole) dissolved in 100.0 grams of ethyl alcohol (C2H5OH, 46.08 g/mole) at 25oC. The vapor pressure of pure water is 23.8 mm Hg, and that of pure ethyl alcohol is 61.2 mm Hg at 25oC. What is the mole fraction of ethyl alcohol in the liquid phase? XEA = 0.610 What is the vapor pressure of ethyl alcohol and the vapor pressure of water above the solution? What is the total pressure of the solution? PEA = 37.3 mmHg PH2O= 9.28 mmHg Psoln = 46.6 mmHg What is the

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