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Midterm I

Midterm I. Thursday, Feb. 08: Huggins 10, 7-8 pm Conflicts: ELL 221, 6-7 pm. Review. No class on Friday, Feb. 09!. 13.26.

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Midterm I

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  1. Midterm I Thursday, Feb. 08: Huggins 10, 7-8 pm Conflicts: ELL 221, 6-7 pm Review No class on Friday, Feb. 09!

  2. 13.26 The solubility of KNO3 is 155 g per 100 g of water at 75oC and 38.0 g at 25oC. What mass (in grams) of KNO3 will crystallize out of solution if exactly 100 g of its saturated solution at 75oC are cooled to 25oC?

  3. Colligative properties Depend on amount of solute in solution, not on identity • Vapor-pressure lowering • Nonvolatile solute, ΔP = X2P1o • Does not apply to volatile solutes, PT = PA + PB (where PA = XAPAo) • Boiling-point elevation • ΔTb = Kbm • Does not apply to volatile solutes • Freezing-point depression • ΔTf = KFm • Applies to both volatile and nonvolatile solutes • Osmotic pressure • Directly proportional to concentration of solution • As [solute] increases, so does osmotic pressure (pressure required to stop osmosis, which is the net movement of solvent molecules from a pure solvent or dilute solution to a more concentrated solution)

  4. Colligative properties Depend on amount of solute in solution, not on identity • You just dissolved a solute such as sugar in water. What happens to each of the following? • Melting point (freezing point) • decreases • Boiling point • increases • Vapor pressure • decreases • Osmotic pressure • increases

  5. Using colligative properties to determine molar mass Example 13.8 A 7.85-g sample of a compound is dissolved in 301 g of benzene. The freezing point of the solution is 1.05oC below that of pure benzene. What is the molar mass of this compound?

  6. Solutions and solubility • In most, but not all cases, solubility of a solid substance increases with temperature • The solubility of gases usually decreases with temperature • The solubility of gases usually increases with pressure • Henry’s law, c = kP

  7. Effect of pressure on the solubility of gases • Henry’s law, c = kP The solubility of N2 in blood at 37oC and at a partial pressure of 0.80 atm is 5.6 x 10-4 mol L-1. What is the value of Henry’s Law constant? In M mmHg-1?

  8. Concentration units • % by mass of solute • Mass of solute/mass of solution, where mass of solution is (mass of solute + mass of solvent) • Molarity, M • Moles of solute/ volume of solution (L) • Molality, m • Moles of solute/ mass of solvent (kg)

  9. Concentration units • % by mass of solute • Mass of solute/mass of solution, where mass of solution is (mass of solute + mass of solvent) • Molarity, M • Moles of solute/ volume of solution (L) • Molality, m • Moles of solute/ mass of solvent (kg) Calculate the molality and molarity of a 30% by mass solution of NH3 in water. The density of the solution is 0.982 g/mL.

  10. The rate constant • The rate constant k for a given reaction depends on what parameters? • The rate constant does NOT depend on the concentrations of reactants!

  11. Rates of reaction • Half-life t1/2 is the time it takes for the concentration of • reactant to decrease to ½ its initial value • For a first-order reaction, how long will it take for the concentration of reactant to fall to 1/8 its original value? • Half-life of a 1st order reaction does not depend on the concentration.

  12. Rates of reaction • Determining reaction order graphically • A plot of ln [A] vs. time gives a straight line • 1st order • A plot of 1/[A] vs. time gives a straight line • 2nd order

  13. Catalysts • Rate constant • Increases • Equilibrium constant • No change

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