Chapter 11, Problem 75AP

Consider two hypothetical pure substances, AB(s) and XY(s). When equal molar amounts of these substances are placed in separate 500-mL samples of water, they undergo the following reactions: AB(s)A+(aq)+B(aq)XY(s)XY(aq) a Which solution would you expect to have the lower boiling point? Why? b Would you expect the vapor pressures of the two solutions to be equal? If not, which one would you expect to have the higher vapor pressure? c Describe a procedure that would make the two solutions have the same boiling point. d If you took 250 mL of the AB(aq) solution prepared above, would it have the same boiling point as the original solution? Be sure to explain your answer. e The container of XY(aq) is left out on the bench top for several days, which allows some of the water to evaporate from the solution. How would the melting point of this solution compare to the melting point of the original solution?

Two samples of sodium chloride solutions are brought to a boil on a stove. One of the solutions boils at 100.10C and the other at 100.15C. a Which of the solutions is more concentrated? b Which of the solutions would have a lower freezing point? c If you split the solution that boils at 100.1C into two portions, how would the boiling points of the samples compare? Which of the following statements do you agree with regarding the determination of your answer for part c? I. The question cannot be answered with certainty without knowing the volumes of each portion. II. Making the necessary assumption that the two samples have equal volumes, I was able to correctly answer the question. III. The volumes that the sample was split into are irrelevant when determining the correct answer.

When KNO3 is dissolved in water, the resulting solution is significantly colder than the water was originally. (a) Is the dissolution of KNO3 an endothermic or an exothermic process? (b) What conclusions can you draw about the intermolecular attractions involved in the process? (c) Is the resulting solution an ideal solution?

Carbon tetrachloride (CCl4) boils at 76.8C and has a density of 1.59 g/mL. (a) A solution prepared by dissolving 0.287 mol of a nonelectrolyte in 255 mL of CCl4 boils at 80.3C. What is the boiling point constant (kb) for CCl4? (b) Another solution is prepared by dissolving 37.1 g of an electrolyte (MM=167g/mol) in 244 mL of CCl4. The resulting solution boils at 85.2C. What is i for the electrolyte?

A 12.0-g sample of a nonelectrolyte is dissolved in 80.0 g of water. The solution freezes at -1.94 C. Calculate the molar mass of the substance.

A forensic chemist is given a white solid that is suspected of being pure cocaine (C17H21NO4, molar mass = 303.35 g/mol). She dissolves 1.22 0.01 g of the solid in 15.60 0.01 g benzene. The freezing point is lowered by 1.32 0.04C. a. What is the molar mass of the substance? Assuming that the percent uncertainty in the calculated molar mass is the same as the percent uncertainty in the temperature change, calculate the uncertainty in the molar mass. b. Could the chemist unequivocally state that the substance is cocaine? For example, is the uncertainty small enough to distinguish cocaine from codeine (C18H21NO3, molar mass = 299.36 g/mol)? c. Assuming that the absolute uncertainties in the measurements of temperature and mass remain unchanged, how could the chemist improve the precision of her results?

Infer Dehydration occurs when more fluid is lost from the body than is taken in. Scuba divers are advised to hydrate their bodies before diving. Use your knowledge of the relationship between pressure and gas solubility to explain the importance of hydration prior to a dive.

Acetone, CH3COCH3, is a common laboratory solvent. It is usually contaminated with water, however. Why does acetone absorb water so readily? Draw molecular structures showing how water and acetone can interact. What intermolecular force(s) is(are) involved in the interaction?

6-93 Two bottles of water are carbonated, with CO2 gas being added, under 2 atm pressure and then capped. One bottle is stored at room temperature; the other is stored in the refrigerator. When the bottle stored at room temperature is opened, large bubbles escape, along with a third of the water. The bottle stored in the refrigerator is opened without frothing or bubbles escaping. Explain.