CHAPTER 8:

LIQUIDS AND SOLUTIONS

Physical Properties of Liquids and Solutions

8-1. Which of the following correctly describes the behavior of the vapor pressure of liquids?

          (a) It remains constant as the temperature of the liquid increases.

          (b) It remains constant when a solute is dissolved in the liquid.

          (c) It increases when a solute is dissolved in the liquid.

(d) If the vapor pressure of CH₃OCH₃ is much larger than CH₃CH₂OH at room temperature, CH₃OCH₃ should boil at a lower temperature than CH₃CH₂OH.

          (e) All of the above are correct.

Answer: (d)

8-2. Which compound would you expect to have the largest vapor pressure at 25°C?

           (a) C₇H₁₆ (BP = 98.4°C)  (b) C₈H₁₈ (BP = 125.7°C)

          (c) C₉H₂₀ (BP = 150.8°C) (d) C₁₀H₂₂ (BP = 174.3°C)

(e) It is impossible to tell which compound would have the largest vapor pressure because there is no relationship between vapor pressure and the boiling point of a compound.

Answer: (a)

8-3. Which of the following would you expect to have the largest boiling point?

          (a) methane, CH₄ (b) chloromethane, CH₃Cl (c) dichloromethane, CH₂Cl₂

(d) chloroform, CHCl₃ (e) carbon tetrachloride, CCl₄

Answer: (e)

8-4. The boiling point of HF is lower than that of water despite the fact that the hydrogen bonds are in HF are at least as strong as those in water because:

          (a) water is more polar than HF    (b) water has the higher melting point

          (c) water is less dense       (d) each water molecule forms more hydrogen bonds

Answer: (d)

8-5. In which of the following substances do you expect to find the strongest hydrogen bonds?

          (a) NH₃ (b) H₂S (c) H₂O (d) CF₄ (e) NaH

Answer: (c)

Units of Concentration

8-6. What is the molality of a bromine solution made by dissolving 39.95 g of Br₂ in 500 g of CCl₄?

          (a) 0.25 (b) 0.50 (c) 1 (d) 2

Answer: (b)

8-7. Which of the following is true?

(a) Molarity (M) is calculated by dividing the number of moles of solute by the number of liters of solvent.

(b) Molality (m) is calculated by dividing the number of moles of solute by the number of liters of solvent.

(c) Mole fraction is calculated by dividing the number of moles of solvent by the number of moles of solute.

(d) Mole fraction is calculated by dividing the number of moles of solute by the number of moles of solvent.

           (e) Statements (a) through (d) are all false.

Answer: (e)

8-8. A 10.0% by weight solution of NaCl (78.5 g/mol) in water has a molality of:

          (a) 0.127 m (b) 1.42 m (c) 1.50 m (d) 7.85 m

          (e) impossible to determine unless you know the density of the solution

Answer: (b)

8-9. Calculate the molarity of a 20.0% by weight solution of CuCl₂ in water if the density of this solution is 1.205 g/cm³.

Answer: 1.79 M

8-10. What is the mole fraction of CCl₄ (MW = 154 g/mol) in a solution prepared by dissolving 32 grams of CCl₄ in 75 grams of C₆H₆?

           (a) 0.18 (b) 0.22 (c) 0.30 (d) 0.82

Answer: (a)

8-11. What volume of a solution of sulfuric acid that is 60.0% by weight H₂SO₄ (MW = 98 g/mol, d = 1.50 g/cm³) would be required to prepare 500 mL of a 1.5 M solution?

Answer: 81.7 mL

8-12. For a solution of 2 g of solute (MW = 200 g/mol) dissolved in 200 g of solvent (MW = 20 g/mol), the mole ratio of solute to solvent is:

          (a) 1 (b) 10^-1 (c) 10^-2 (d) 10^-3 (e) 10^-4

Answer: (d)

8-13. Which of the following concentrations units would change when the temperature of the solution changes? (Hint: remember that liquids expand and contract as the temperature changes.)

                     (I) molarity (II) molality (III) mole fraction of solute

                     (IV) mole fraction of solvent

          (a) I (b) I and II (c) I, II and III

          (d) all of the above (e) none of the above

Answer: (a)

8-14. Which of the following concentration units would all be independent of temperature?           (a) molality and mole fraction        (b) molality, molarity, and mole fraction

          (c) molarity and molality             (d) molality, mole fraction, and percent by weight

Answer: (d)

Phase Diagrams

8-15. Which of the following is correct?

(a) Solids melt at their triple point (the temperature at which the gas, liquid and solid are all in equilibrium).

(b) Liquids can't boil unless the vapor pressure of the liquid is equal to one atmosphere pressure (760 mmHg).

(c) Solutions boil at a higher temperature than the pure solvent because we have to heat the solution to a higher temperature before atmospheric pressure is equal to 760 mmHg.

(d) Solutions have a larger vapor pressure than the pure solvent.

(e) None of the above are correct.

Answer: (e)

8-16. Reading from left to right across a phase diagram, what is the order in which you would find the three states of matter?

          (a) gas < liquid < solid (b) gas < solid < liquid (c) liquid < gas < solid

          (d) liquid < solid < gas (e) solid < liquid < gas

Answer: (e)

8-17. Which statement isn’t consistent with the information contained in the phase diagram of a compound?

(a) Liquids can be made to boil by raising the pressure at constant temperature.

(b) Solids can be made to melt by raising the temperature at constant pressure.

(c) Gases are most likely to be found at high temperatures and low pressures.

(d) The boiling point of a compound is the temperature at which the vapor pressure of the compound is equal to atmospheric pressure

(e) All of these statements are consistent with the information in the phase diagram of a compound.

Answer: (a)

8-18. Gases are most likely to be found in a phase diagram under conditions of:

          (a) high temperature and high pressure. (b) high temperature and low pressure.           (c) low temperature and high pressure. (d) low temperature and low pressure.

          (e) moderate temperature and pressure.

Answer: (b)

Solubility

8-19. Which of the following will be most soluble in a nonpolar solvent such as carbon tetrachloride, CCl₄?

          (a) KI (b) H₂O (c) NH₃ (d) CBr₄ (e) HF

Answer: (d)

8-20. Which alcohol should be most soluble in a nonpolar solvent such as benzene, C₆H₆?           (a) CH₃OH (b) CH₃CH₂OH (c) CH₃CH₂CH₂OH

(d) CH₃CH₂CH₂CH₂OH (e) CH₃CH₂CH₂CH₂CH₂OH

Answer: (e)

8-21. Carboxylic acids with the general formula CH₃(CH₂)_nCO₂H have a nonpolar CH₃-CH₂... tail and a polar CO₂H head. What effect would increasing the value of "n" have on the solubility of carboxylic acids?

          (a) the solubility would increase in both water and in nonpolar solvents such as CCl₄

(b) the solubility would decrease in both water and in nonpolar solvents such as CCl₄.

(c) the solubility would increase in water but decrease in CCl₄.

           (d) the solubility would decrease in water but increase in CCl₄.

          (e) the solubility of carboxylic acids in both water and CCl₄ would remain the same.

Answer: (d)

8-22. Which of the following statements is correct for the solubilities of alcohols with the formula CH₃(CH₂)_nOH?

          (a) the solubility in water increases as n increases.

           (b) the solubility in water decreases as n increases.

          (c) the solubility in water is independent of n.

          (d) the solubility in water depends only on the extent of hydrogen bonding.

Answer: (b)

8-23. An alcohol is a compound with the formula CH₃(CH₂)_nOH, in which the CH₂(CH₂)_n group is nonpolar and the OH group is polar. As "n" increases for different alcohols, we would expect the solubilities of the alcohol to:

          (a) increase in both polar and nonpolar solvents.

          (b) increase in polar solvents and decrease in nonpolar solvents.

          (c) decrease in both polar and nonpolar solvents.

           (d) decrease in polar solvents and increase in nonpolar solvents.

Answer: (d)

8-24. The best explanation for the fact that water and ethyl alcohol are completely miscible is that:

          (a) both compounds are nonpolar.  

(b) water is polar and ethyl alcohol is nonpolar.

          (c) water has an unusually large dielectric constant.

           (d) hydrogen bonding takes place between the two solvents.

Answer: (d)

8-25. Potassium iodide reacts with iodine in aqueous solution to form the triiodide ion:

KI(aq) + I₂(aq) ➝ KI₃(aq)

What would happen if we added CCl₄ to this reaction?

          (a) The KI would tend to dissolve in the CCl₄ layer.

           (b) The I₂ would tend to dissolve in the CCl₄ layer.

          (c) Both KI and I₂ would dissolve in the CCl₄ layer.

          (d) Neither KI nor I₂ would dissolve in the CCl₄ layer.

          (e) Because CCl₄ and water are miscible, no distinct CCl₄ layer would form.

Answer: (b)

8-26. The equilibrium constant for the following reaction can be measured by extracting this solution with a nonpolar solvent and then titrating both the nonpolar solvent and the aqueous solution with sodium thiosulfate.

I₂(aq) + I^-(aq) ➝ I₃^-(aq)

Which of the following dissolves in the nonpolar solvent?

(a) I₃^- (b) I^- (c) I₃^- and I^- (d) I₂ (e) I₂, I^- and I₃^-

Answer: (d)

Colligative Properties

8-27. Which of the following liquid systems would be expected to have the lowest vapor pressure at room temperature?

          (a) a 1.00 m solution of alcohol in water

          (b) a 1.00 m solution of sugar in water

           (c) a 1.00 m solution of NaCl in water  

          (d) pure water

Answer: (c)

8-28. Raoult's law predicts that the vapor pressure of solvent over a solution of a nonvolatile solute:

(a) Increases with increasing mole fraction of solvent

(b) Increases with increasing mole fraction of solute

          (c) Is independent of the solute concentration

          (d) Decreases with increasing temperature.

Answer: (a)

8-29. When a nonvolatile solute is dissolved in a volatile solvent, the vapor pressure of the solvent over the solution:

          (a) increases as the mole fraction of solute increases.

           (b) decreases as the mole fraction of solute increases.

          (c) is independent of the mole fraction of solute.

          (d) changes in an unpredictable way with changes in the mole fraction of the solute.

Answer: (b)

8-30. For a nonvolatile solute dissolved in a volatile solvent, the vapor pressure, freezing point, and boiling point:

          (a) are all higher for the solution than for the pure solvent.

          (b) are all lower for the solution than for the pure solvent.

(c) change differently, with vapor pressure increasing and boiling and freezing points decreasing.

(d) change differently, with boiling point increasing and vapor pressure and freezing point decreasing.

Answer: (d)

8-31. For a solution of two volatile compounds, the ratio of the vapor pressure of the two compounds in the gas phase over the liquid is:

          (a) Proportional to the ratio of the vapor pressures of the pure compounds.

          (b) Proportional to the sum of the vapor pressures of the pure compounds.

          (c) Independent of the vapor pressures of the pure compounds.

          (d) Proportional to the sum of the mole fractions of the two components.

           (e) Proportional to the ratio of the mole fractions of the two components.

Answer: (e)

8-32. Given dilute, equimolar solutions of NaCl and ZnCl₂ in water, we would expect the freezing points for the solutions relative to pure water to:

          (a) change by the same amount.

           (b) change by a ratio of 2:3, with the change for NaCl being 2/3rds that for ZnCl₂.

          (c) change by a ratio of 1:3, with the change for NaCl being 1/3rd that for ZnCl₂.

          (d) change by a ratio of 2:3, with the change for ZnCl₂ being 2/3rds that of NaCl.

          (e) change by a ratio of 1:3, with the change for ZnCl₂ being 1/3rd that for ZnCl.

Answer: (b)

8-33. If freezing-point depression was used to determine the molecular weight of an acid dissolved in water, and the dissociation of the acid wasn’t taken into account, the determined value of the molecular weight of the acid would be:

          (a) accurate (b) too high (c) too low

          (d) inaccurate, but in a direction that cannot be predicted.

Answer: (c)

          [Note: 42% of the sample chose the wrong answer: (b)]

8-34. Which of the following equations would be appropriate for calculating the change in the freezing point of an aqueous NaHSO₄ solution. NaHSO₄ dissociates in solution in a two-step process.

NaHSO₄(s) ➝ Na⁺(aq) + HSO₄^-(aq)(100% dissociation)

HSO₄^-(aq) ➝ H⁺(aq) + SO₄^2-(aq)(10% dissociation)

          (a) ΔT_f = (1 + 0.10)k_fm        (b) ΔT_f = (0.10)k_fm

          (c) ΔT_f = [1 + 2( 0.10)]k_fm   (d) ΔT_f = (0.20)k_fm

           (e) ΔT_f = (2 + 0.10)k_fm 

Answer: (e)

8-35. An 0.010 molal solution of ammonia in water is 4.1% ionized.

          NH₃(aq) + H₂O(l) ➝ NH₄⁺(aq) + OH^-(aq)

Given that pure water freezes at 0°C and the k_F for water is 1.86°C/m, which of the following expressions gives the freezing point depression of the aqueous ammonia solution?

          (a) ΔT_F = -(1.86 °C/m)(0.01 m) 

           (b) ΔT_F = -(1.86 °C/m)(0.01 m)(1.041)

          (c) ΔT_F = -(1.86 °C/m)(0.01 m)(2.082)

          (d) ΔT_F = -(1.86 °C/m)(0.01 m)(4.1)

          (e) ΔT_F = -(1.86 °C/m)(0.01 m)(8.2)

Answer: (b)

8-36. Which of the following correctly describes a graph of the freezing point of a solution versus the molality of the solution?

          (a) The freezing point increases linearly with the molality of the solution.

           (b) The freezing point decreases linearly with the molality of the solution.

          (c) The freezing point remains constant as the molality of the solution increases.

          (d) The freezing point increases exponentially with the molality of the solution.

          (e) The freezing point decreases exponentially the molality of the solution.

Answer: (b)

8-37. An 0.100 molal solution of sulfuric acid in water freezes at -0.371°C. Which of the following statements agrees with this experimental observation? (k_f for H₂O = -1.853 °C/molal)

(a) H₂SO₄ doesn’t dissociate in water.

           (b) H₂SO₄ dissociates into H₃O⁺ and HSO₄^- ions in water.

          (c) H₂SO₄ dissociates in water to form two H₃O⁺ ions and one SO₄^2- ion.

          (d) H₂SO₄ associates in water to form (H₂SO₄)₂ dimers.

          (e) None of the above are consistent with the experimental facts.

Answer: (b)

8-38. KCl is a strong electrolyte, while sucrose (table sugar) is not. Which of the following lists 0.10 M KCl, 0.10 M sucrose, and pure water in order of increasing boiling point?

          (a) KCl < sucrose < water          (b) water < KCl = sucrose

          (c) KCl = sucrose < water         (d) water < sucrose < KCl

          (e) sucrose < KCl < water

Answer: (d)

Colligative Properties Calculations

8-39. 0.400 mole of CCl₄ and 0.600 mole of CHCl₃ are mixed at 43°C. The vapor pressure of pure CCl₄ at this temperature is 0.354 atm and the vapor pressure of pure CHCl₃ at this temperature is 0.526 atm. What is the vapor pressure of the solution?

          (a) 0.285 atm (b) 0.423 atm (c) 0.457 atm (d) 0.526 atm (e) 0.880 atm

Answer: (c)

8-40. What would be the freezing point of a saturated solution of caffeine (C₈H₁₀O₂N₄) in water if it takes 45.6 grams of water to dissolve 1.00 gram of caffeine? (caffeine: MW = 194.19; H₂O: k_f = 1.853 °C/molal)

Answer: -0.209°C

8-41.The "Tip of the Week" in a recent newspaper suggested using a fertilizer such as ammonium nitrate or ammonium sulfate instead of salt to melt snow and ice on sidewalks because salt can damage lawns. Which of the following compounds would give the largest freezing point depression when 100 grams of the compound are dissolved in 1 kg of water?        (a) NaCl (58.44 g/mol) (b) NH₄NO₃ (80.05 g/mol) (c) (NH₄)₂SO₄ (132.14 g/mol)

          (d) These solutions would all have the same freezing point depression.

Answer: (a)

8-42. The melting point of pure benzene is 278.70 K and the molal freezing point depression constant is 4.90 K/molal. When 4.20 g of an unknown solute is added to 100 g of benzene, the freezing point of the solution is 277.60 K. What is the molecular weight of the unknown?

          (a) 12.7 g/mol (b) 18.7 g/mol (c) 128 g/mol

           (d) 187 g/mol (e) 226 g/mol

Answer: (d)

8-43. If 5.17 grams of an unknown substance X that is a nonelectrolyte lowers the freezing point of 200 g of benzene (k_f = 4.90 K/m) by 0.84 K, what is the molecular weight of X?

          (a) 4.26 (b) 6.67 (c) 44.3 (d) 151 (e) 426 g/mol

Answer: (d)

8-44. When a 2.15 gram sample of a nonelectrolyte is dissolved in 105 grams of water, the freezing point of the solution is -0.62°C. If the molal freezing point depression constant for water is 1.86°C/m, what is the molecular weight of the nonelectrolyte?

Answer: 61.4 g/mol

8-45.The molal freezing point constant for water is 1.86 °C/m and the freezing point for pure water is 0°C. When 1.50 g of a compound is added to 47.8 g of water, the freezing point of the solution is found to change by 0.174°C. Calculate the molecular weight of the solute.

Answer: 335 g/mol

8-46. Pure benzene melts at 5.5°C. A solution of 1.25 g of CCl₄ in 100 g of benzene would have what freezing point? (Benzene: k_b = 5.1°C/m)

Answer: 5.09°C

8-47. Pure ethanol has a boiling point of 78.0°C. What is the boiling point of a solution that contains 12.7 grams of KI (MW = 166 g/mol) dissolved in 125 grams of ethanol (MW = 46.0 g/mol). Assume that KI is a strong electrolyte in ethanol. (Ethanol: k_b = 1.22°C/m)

(a) 76.5°C (b) 77.2C (c) 78.7°C (d) 79.5°C

Answer: (d)

8-48. Which of the following solutions has the highest boiling point?

          (a) Water saturated with N₂            (b) 0.1 M sucrose

           (c) 0.1 M Na₂CO₂                          (d) 0.1 M NH₃

           (e) 0.1 M NH₄Cl

Answer: (c)

8-49. Explain the following observation: A large amount of water is lost by fruit when sugar is sprinkled on the fruit.