Chapter 11 Problems

1. If `DeltaH_"solution" = 230 J"/"mol`, `DeltaH_"solvent" = 108 J"/"mol`, and `DeltaH_"mixing" = -47 J"/"mol`, what is the value of `H_"solute"`?

2. If `DeltaH_"solution" = -219 J"/"mol`, is the solution ideal or nonideal? The change in energy accompanying the process

            (a) heat of hydration    (d) heat of ionization

            (b) heat of solution      (e) heat of dissociation

            (c) crystal lattice energy of MX

3. Would each of the following solutions be expected to exist? Why or why not?

(a)     `MgCl_2 " in " C Cl_4`                               (b)  `NaC l` in water

(b)     Pentane, `C_5 H_(12)`, in `C Cl_4`                 (d)  methanol, `C H_3OH`, in water

(e)  Vegetable oil in mineral oil

 

5.  Select the most appropriate explanation of the following observation on solubility:

            "Hydrogen chloride, `HCl`, is very soluble in water."

(a)    Water is a polar solvent, and it promotes the ionization of many polar molecules.

(b)    Opposites attract, that is, polar solutes dissolve in non-polar solvents and vice- versa.

(c)    Water promotes the dissociation of many ionic solids.

(d)    Gases that interact only slightly with solvents dissolve freely.

(e)    Relatively light molecules are generally more soluble in water than heavier molecules.

 

6. Using the theory that 'like dissolves like' pick an appropriate solvent from the following table to dissolve each of the following. State the kind of intermolecular forces that would occur between the solute and solvent in each case:

Common Polar Solvents	Common Non-polar Solvents
Water (`H_2O`)	Hexane (`C_6H_(14)`)
Acetone (`CH_3COCH_3`)	Diethyl ether (CH3CH­2OCH2CH3)*
Methanol (`CH_3OH`)	Toluene (`C_7H_8`)
Ethanol (`CH_3CH_2OH`)	Carbon tetrachloride (`CC l_4`)

* Diethyl ether can be considered to be intermediate between polar and non polar

a) motor oil (non polar)

b) lard 

c) potassium chloride

d) naphthalene

 

7. A person preparing a fish tank uses boiled (and then cooled) water to fill it. When the fish is put into the tank, it dies. Explain   why this happens using what you know about solutions.

 

8. Use Henry's law to determine the molar solubility of He at a pressure of 1.0 atm and 25°C.

 

9. A solution is prepared by dissolving` 28.4 g` of glucose (`C_6H_(12)O_6`) in `355 g` of water at 25°C. The final volume of the solution is `378 mL`.  For this solution, calculate the following: a) molality and b) mole fraction.

 

10. 1.5 moles of cherry Kool-Aid are added to a pitcher containing 2 liters of water on a nice day at 25^o C. The vapor pressure of water alone is 23.8 mm Hg at 25^o C. What is the new vapor pressure of Kool-Aid?

 

11. What are the partial pressures of benzene and toluene in a solution in which the mole fraction of benzene is 0.6? What is the total vapor pressure? The vapor pressure of pure benzene is 95.1 mm `Hg` and the vapor pressure of pure toluene 28.4 mm Hg at 25^oC.

 

12. Predict the boiling point of a 1.25m aqueous sucrose solution. Sucrose is a nonvolatile, nonionized solute.

 

13. A solution contains 8.92 g of `KBr` in `500.0 mL` of solution and has an osmotic pressure of 6.97 atm at 25°C. Calculate the van't Hoff factor (i) for `KBr` at this concentration. (This question is practice for Osmotic pressure and van't Hoff.)

 

14. Colloids are described by all of the following except:

(a)    The particles in a colloid are so small that settling is negligible.

(b)    The mixture appears cloudy.

(c)    Only combinations of liquids and gases can form colloids.

(d)    Colloids are not suspensions or homogeneous mixtures.

(e)    Mayonnaise, whipped cream and fog are all examples of colloids.

 

15. When lithium iodide (LiI) is dissolved in water, the solution becomes hotter. a) Is the dissolution of lithium iodide endothermic or exothermic? b) Sketch an energy diagram for the dissolution of LiI. This should include the heat of hydration, lattice energy, and heat of solution. c) What can you say about the relative magnitudes of the lattice energy of lithium iodide and its heat of hydration?

16. Using the table in questions 6, pic an appropriate solvent to dissolve: a) isopropyl alcohol, b) sodium chloride, c) dimethyl ether, d) ethylene glycol.

17. Scuba divers breathing air at increased pressure can suffer from oxygen toxicity - too much oxygen in the bloodstream - when the partial pressure of oxygen exceeds about 1.4 atm. What happens to the amount of oxygen in a diver's bloodstream when he or she breathes oxygen at elevated pressures? How can this be reversed?

18. Use Henry's Law to determine the molar solubility of Ar at a pressure of 1.3 atm and 25°C.

19. A solution is prepared by dissolving 20.2 mL of methanol (`CH_3OH`) in 100.0 mL of water at 25°C. The final volume of the solution is 118 mL. The densities of methanol and water at this temperature are 0.782 g/mL and 1.00 g/mL, respectively. For this solution, calculate the following: a) molality and b) mole fraction.

20. What is the vapor pressure of pure butane at 20^oC if its partial pressure is 698 mm Hg in a butane-acetone mixture where the mole fraction of acetone is x = 0.577?

21. A glucose solution contains 55.8 g of glucose (`C_6H_(12)O_6`) in 455g water. Compute the freezing point and boiling point of the solution.

22. A 0.100 M ionic solution has an osmotic pressure of 8.3 atm at 25°C. Calculate the van't Hoff factor (i) for this solution.