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Questions and Problems
NaCl solution placed on top of the mercury. Identify X, Y, and Z with these solutions. X
Y
Z
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13.90 13.84 A forensic chemist is given a white powder for analysis. She dissolves 0.50 g of the substance in 8.0 g of benzene. The solution freezes at 3.9°C. Can the chemist conclude that the compound is cocaine (C17H21NO4)? What assumptions are made in the analysis? 13.85 “Time-release” drugs have the advantage of releasing the drug to the body at a constant rate so that the drug concentration at any time is not so high as to have harmful side effects or so low as to be ineffective. A schematic diagram of a pill that works on this basis is shown here. Explain how it works.
Semipermeable membrane
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Elastic impermeable membrane Saturated NaCl solution
Drug
Rigid wall containing tiny holes
13.86 Concentrated hydrochloric acid is usually available at 37.7 percent by mass. What is its concentration in molarity? (The density of the solution is 1.19 g/mL.) 13.87 A protein has been isolated as a salt with the formula Na20P (this notation means that there are 20 Na1 ions associated with a negatively charged protein P202). The osmotic pressure of a 10.0-mL solution containing 0.225 g of the protein is 0.257 atm at 25.0°C. (a) Calculate the molar mass of the protein from these data. (b) What is the actual molar mass of the protein? 13.88 A nonvolatile organic compound Z was used to make two solutions. Solution A contains 5.00 g of Z
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dissolved in 100 g of water, and solution B contains 2.31 g of Z dissolved in 100 g of benzene. Solution A has a vapor pressure of 754.5 mmHg at the normal boiling point of water, and solution B has the same vapor pressure at the normal boiling point of benzene. Calculate the molar mass of Z in solutions A and B and account for the difference. Hydrogen peroxide with a concentration of 3.0 percent (3.0 g of H2O2 in 100 mL of solution) is sold in drugstores for use as an antiseptic. For a 10.0-mL 3.0 percent H2O2 solution, calculate (a) the oxygen gas produced (in liters) at STP when the compound undergoes complete decomposition and (b) the ratio of the volume of O2 collected to the initial volume of the H2O2 solution. Before a carbonated beverage bottle is sealed, it is pressurized with a mixture of air and carbon dioxide. (a) Explain the effervescence that occurs when the cap of the bottle is removed. (b) What causes the fog to form near the mouth of the bottle right after the cap is removed? Two beakers, one containing a 50-mL aqueous 1.0 M glucose solution and the other a 50-mL aqueous 2.0 M glucose solution, are placed under a tightly sealed bell jar as that shown in Figure 13.9 at room temperature. What are the volumes in these two beakers at equilibrium? Assume ideal behavior. Explain each of these statements: (a) The boiling point of seawater is higher than that of pure water. (b) Carbon dioxide escapes from the solution when the cap is removed from a soft-drink bottle. (c) Molal concentrations and molar concentrations of dilute aqueous solutions are approximately equal. (d) In discussing the colligative properties of a solution (other than osmotic pressure), it is preferable to express the concentration in units of molality rather than in molarity. (e) Methanol (b.p. 65°C) is useful as an auto antifreeze, but it should be removed from the car radiator during the summer season. Acetic acid is a weak acid that ionizes in solution as follows: CH3COOH(aq) Δ CH3COO2 (aq) 1 H1 (aq)
If the freezing point of a 0.106 m CH3COOH solution is 20.203°C, calculate the percent of the acid that has undergone ionization. 13.94 A 1.32-g sample of a mixture of cyclohexane (C6H12) and naphthalene (C10H8) is dissolved in 18.9 g of benzene (C6H6). The freezing point of the solution is 2.2°C. Calculate the mass percent of the mixture. 13.95 How does each of the following affect the solubility of an ionic compound: (a) lattice energy, (b) solvent (polar versus nonpolar), (c) enthalpies of hydration of cation and anion?