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Solutions

Solutions. Why does a raw egg swell or shrink when placed in different solutions?. Some Definitions. A solution is a HOMOGENEOUS mixture of 2 or more substances in a single phase. One constituent is usually regarded as the SOLVENT and the others as SOLUTES. Definitions.

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Solutions

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  1. Solutions Why does a raw egg swell or shrink when placed in different solutions?

  2. Some Definitions A solution is a HOMOGENEOUSmixture of 2 or more substances in a single phase. One constituent is usually regarded as the SOLVENTand the others as SOLUTES.

  3. Definitions Solutions can be classified as saturated or unsaturated. A saturated solution contains the maximum quantity of solute that dissolves at that temperature. SUPERSATURATED SOLUTIONS contain more than is possible and are unstable.

  4. Dissolving An Ionic Solid Active Figure 14.9

  5. Energetics of the Solution Process Figure 14.8

  6. Energetics of the Solution Process If the enthalpy of formation of the solution is more negative that that of the solvent and solute, the enthalpy of solution is negative. The solution process is exothermic!

  7. Supersaturated Sodium Acetate • One application of a supersaturated solution is the sodium acetate “heat pack.” • Sodium acetate has an ENDOthermic heat of solution.

  8. Supersaturated Sodium Acetate Sodium acetate has an ENDOthermic heat of solution. NaCH3CO2 (s) + heat ----> Na+(aq) + CH3CO2-(aq) Therefore, formation of solid sodium acetate from its ions is EXOTHERMIC. Na+(aq) + CH3CO2-(aq) ---> NaCH3CO2 (s) + heat

  9. Colligative Properties On adding a solute to a solvent, the properties of the solvent are modified. • Vapor pressure decreases • Melting point decreases • Boiling point increases • Osmosis is possible (osmotic pressure) These changes are called COLLIGATIVE PROPERTIES. They depend only on the NUMBER of solute particles relative to solvent particles, not on the KIND of solute particles.

  10. Concentration Units An IDEAL SOLUTION is one where the properties depend only on the concentration of solute. Need concentration units to tell us the number of solute particles per solvent particle. The unit “molarity” does not do this!

  11. Concentration Units MOLE FRACTION, X For a mixture of A, B, and C MOLALITY, m WEIGHT % = grams solute per 100 g solution (also ppm, ppb, which is like %, which is parts per hundred)

  12. Calculating Concentrations Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O. Calculate mol fraction, molality, and weight % of ethylene glycol.

  13. Calculating Concentrations Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O. Calculate X, m, and % of glycol. 250. g H2O = 13.9 mol Calculate mole fraction X glycol = 0.0672

  14. Calculating Concentrations Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O. Calculate X, m, and % of glycol. Calculate molality Calculate weight %

  15. Dissolving Gases & Henry’s Law Sg Gas solubility (mol/L) = kH • Pgas kH for O2 = 1.66 x 10-6 M/mmHg When Pgas drops, solubility drops.

  16. Understanding Colligative Properties To understand colligative properties, study the LIQUID-VAPOR EQUILIBRIUM for a solution.

  17. Understanding Colligative Properties Vapor pressure of H2O over a solution depends on the number of H2O molecules per solute molecule. Psolventproportional to Xsolvent Psolvent = Xsolvent • Posolvent VP of solvent over solution = (Mol frac solvent)•(VP pure solvent) RAOULT’S LAW

  18. Raoult’s Law An ideal solution is one that obeys Raoult’s law. PA = XA • PoA Because mole fraction of solvent, XA, is always less than 1, then PA is always less than PoA. The vapor pressure of solvent over a solution is always LOWERED!

  19. Vapor Pressure Lowering Figure 14.14

  20. Raoult’s Law Assume the solution containing 62.1 g of glycol in 250. g of water is ideal. What is the vapor pressure of water over the solution at 30 oC? (The VP of pure H2O is 31.8 mm Hg) Solution Xglycol = 0.0672 and so Xwater = ? Because Xglycol + Xwater = 1 Xwater = 1.000 - 0.0672 = 0.9328 Pwater = Xwater • Powater = (0.9382)(31.8 mm Hg) Pwater = 29.7 mm Hg

  21. Elevation of Boiling Point Elevation in BP = ∆TBP = KBP•m (where KBP is characteristic of solvent) The boiling point of a solution is higher than that of the pure solvent.

  22. Change in Boiling Point Dissolve 62.1 g of glycol (1.00 mol) in 250. g of water. What is the BP of the solution? KBP = +0.512 oC/molal for water (see Table 14.3). Solution 1. Calculate solution molality = 4.00 m 2. ∆TBP = KBP • m ∆TBP = +0.512 oC/molal (4.00 molal) ∆TBP = +2.05 oC BP = 102.05 oC

  23. Change in Freezing Point Ethylene glycol/water solution Pure water The freezing point of a solution is LOWERthan that of the pure solvent. FP depression = ∆TFP = KFP•m

  24. Lowering the Freezing Point Water with and without antifreeze When a solution freezes, the solid phase is pure water. The solution becomes more concentrated.

  25. Freezing Point Depression How much NaCl must be dissolved in 4.00 kg of water to lower FP to -10.00 oC?. Solution Calc. required molality (*This one is a little different because the solute is ionic, not molecular.) ∆TFP = KFP • m -10.00 oC = (-1.86 oC/molal) • Conc Conc = 5.38 molal

  26. Freezing Point Depression How much NaCl must be dissolved in 4.00 kg of water to lower FP to -10.00 oC?. Solution Concreq’d = 5.38 molal This means we need 5.38 mol of dissolved particles per kg of solvent. *Recognize that m represents the total concentration of all dissolved particles. Recall that 1 molNaCl(aq)--> 1 mol Na+(aq) + 1 molCl-(aq)

  27. Freezing Point Depression How much NaCl must be dissolved in 4.00 kg of water to lower FP to -10.00 oC?. Solution Conc req’d = 5.38 molal We need 5.38 mol of dissolved particles per kg of solvent. Each mol of NaCl contributes 2 mol of dissolved particles. NaCl(aq) --> Na+(aq) + Cl-(aq) To get 5.38 mol/kg of particles we need 5.38 mol / 2 = 2.69 mol NaCl / kg 2.69 mol NaCl / kg ---> 157 g NaCl / kg (157 g NaCl / kg)•(4.00 kg) = 629 g NaCl

  28. Boiling Point Elevation and Freezing Point Depression ∆T = K•m•i A generally useful equation i = van’t Hoff factor = number of particles produced per formula unit. Compound Theoretical Value of i glycol 1 NaCl 2 CaCl2 3

  29. Osmosis Dissolving the shell in vinegar Egg in pure water Egg in corn syrup

  30. Osmosis The semipermeable membrane allows only the movement of solvent molecules. Solvent molecules move from pure solvent to solution in an attempt to make both have the same concentration of solute. Driving force is entropy

  31. Process of Osmosis

  32. Osmosis at the Particulate Level Figure 14.17

  33. Osmosis at the Particulate Level Figure 14.17

  34. Osmosis • Osmosis of solvent from one solution to another can continue until the solutions are ISOTONIC— they have the same concentration.

  35. Osmosis and Living Cells

  36. Reverse OsmosisWater Desalination Water desalination plant in Tampa

  37. Osmosis Calculating a Molar Mass Dissolve 35.0 g of hemoglobin in enough water to make 1.00 L of solution. ∏ measured to be 10.0 mm Hg at 25 ˚C. Calc. molar mass of hemoglobin. Solution (a) Calc. ∏ in atmospheres ∏ = 10.0 mmHg • (1 atm / 760 mmHg) = 0.0132 atm (b) Calculate molarity

  38. Osmosis Calculating a Molar Mass Dissolve 35.0 g of hemoglobin in enough water to make 1.00 L of solution. ∏ measured to be 10.0 mm Hg at 25 ˚C. Calc. molar mass of hemoglobin. Solution (b) Calc. molarity from ∏ = iMRT Conc = 5.39 x 10-4 mol/L (c) Calc. molar mass Molar mass = 35.0 g / 5.39 x 10-4 mol/L Molar mass = 65,100 g/mol

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