exam 1

September 15, 2004

for questions and answers, click here

To receive full credit, you must generally show the work that justifies your answer.

• Claussius Capeyron Equation: ln(P) = - (DHvap/R) (1/T) + C
  • data: R = 8.314 J/K mol R=0.08205 L atm/mol K
• For water the molal freezing point and boiling constants are
  • K_f = 0.51 ^oC kg/mol K_b=1.86 ^oC kg/mol
  • Equilibrium vapor pressure, P(25^oC) = 23.75 mm Hg (or torr)

• 1. (30 points) A solution is made by dissolving
  • 43.85 g of NaCl (that's 0.75 moles)
  • in 500. g.of Water (that's 27.78 moles)
    • The final solution has a volume of 501.5 ml.
    • (Other needed data is at the top of the exam.)
    • hint: NaCl is an ionic salt
  • a. (5 pts) what is the molarity of the solution ?
  • b. (5 pts) what is the molality of the solution ?
  • c. (5 pts) what is the mole fraction of water in the solution?
  • d. (7 pts) what is the freezing point of this solution?
  • e. (8 pts) what is the vapor pressure of water over this solution at 25^oC?

• 2. ( 10 points) The graph shows the vapor pre ssure of two compounds as a function of temperature.
  • Which species has the larger enthalpy of vaporization and what is the basis of your answer?

• 3. (14 points) The next figure shows a phase diagram .
  • a. What phase is present at A?
  • b. What process does the arrow labeled B illustrate?
  • c. what is the name given to the point labeled "C"
  • d. what is the normal boiling point ?(approximately)

• 4. (14 points) What kind of interactions occur between molecules (of the same type)?
  • (List only the strongest forces for each example.)
  • a. Ammonia, NH₃ (that is, interactions between ammonia molecules)
  • b. Benzene (no dipole moment)
  • c. Vinyl Chloride H₂C=CHCl
  • d. on the drawing, indicate a dipole

• 5. (18 points) How much energy is required to melt 36.0 g (2 moles) of ice (0^oC)
• and heat it to 100^oC (without actually boiling it)
  • DH_vap = 40.67 kJ/mol
  • DH_fusion = 6.01 kJ/mol
  • heat capacity of ice = 36.57 J/^oC mol
  • heat capacity of liquid water = 75.3 J/^oC mol
  • heat capacity of steam = 33.6 J/^oC mol

• 6. (14 points) brief answers
  • a. Osmosis involves a semipermeable membrane. What does the semi refer to in the term semipermeable?

• b. Although burning H₂ for energy only produces H₂O, most uses of Hydrogen as a fuel continue to contribute to CO₂ and global warming? Why?
• c. Henry's law could be used to increase the concentration of CO₂ in ordinary water. What would you do to increase the concentration of CO₂ ?)

• d. Define supersaturation, supercooling or superheating.