Questions and Problems About Reactions in Aqueous Solutions
1. Classify these reactions according to the types discussed in the chapter:
(a) Cl2 + 2OH- → Cl- + ClO- + H2O
(b) Ca2+ + CO32- → CaCO3
(c) NH3 + H+ → NH4+
(d) 2CCl4 + CrO42- → 2COCl2 + CrO2Cl2 + 2Cl-
(e) Ca + F2 → CaF2
(f) 2Li + H2 → 2LiH
(g) Ba(NO3)2 + Na2SO4 → 2NaNO3 + BaSO4
(h) CuO + H2 → Cu + H2O
(i) Zn + 2HCl → ZnCl2 + H2
(j) 2FeCl2 + Cl2 → 2FeCl3
2. Someone gave you a colorless liquid. Describe three chemical tests you would perform on the liquid to show that it is water.
3. You are given two colorless solutions, one containing NaCl and the other sucrose (C12H22O11). Suggest a chemical and a physical test that would distinguish between these two solutions. (Physical test: Only the NaCl solution would conduct electricity. Chemical test: Add AgNO3 solution. Only the NaCl solution would give AgCl precipitate.)
4. Chlorine (Cl2) is used to purify drinking water. Too much chlorine is harmful to humans. The excess chlorine is often removed by treatment with sulfur dioxide (SO2). Balance the following equation that represents this procedure:
Cl2 + SO2 + H2O → Cl- + SO42- + H+
5. Before aluminum was obtained by electrolytic reduction from its ore (Al2O3), the metal was produced by chemical reduction of AlCl3. Which metals would you use to reduce Al3+ to Al? (Mg, Na, Ca, Ba, K, or Li).
6. Oxygen (O2) and carbon dioxide (CO2) are colorless and odorless gases. Suggest two chemical tests that would enable you to distinguish between them.
7. Based on oxidation number, explain why carbon monoxide (CO) is fl ammable but carbon dioxide (CO2) is not. (Oxidation number of C is +2 in CO and +4 (maximum) in CO2.)
8. Which of these aqueous solutions would you expect to be the best conductor of electricity at 25°C? Explain your answer.
(a) 0.20 M NaCl
(b) 0.60 M CH3COOH
(c) 0.25 M HCl
(d) 0.20 M Mg(NO3)2
9. A 5.00 x 102 mL sample of 2.00 M HCl solution is treated with 4.47 g of magnesium. Calculate the concentration of the acid solution after all the metal has reacted. Assume that the volume remains unchanged. (1.26 M)
10. Calculate the volume (in liters) of a 0.156 M CuSO4 solution that would react with 7.89 g of zinc.
11. Sodium carbonate (Na2CO3) can be obtained in very pure form and can be used to standardize acid solutions. What is the molarity of an HCl solution if 28.3 mL of the solution is required to react with 0.256 g of Na2CO3? (0.171 M)
12. A 3.664-g sample of a monoprotic acid was dissolved in water and required 20.27 mL of a 0.1578 M NaOH solution for neutralization. Calculate the molar mass of the acid.
13. Acetic acid (CH3COOH) is an important ingredient of vinegar. A sample of 50.0 mL of a commercial vinegar is titrated against a 1.00 M NaOH solution. What is the concentration (in M) of acetic acid present in the vinegar if 5.75 mL of the base were required for the titration? (0.115 M)
14. Calculate the mass of precipitate formed when 2.27 L of 0.0820 M Ba(OH)2 are mixed with 3.06 L of 0.0664 M Na2SO4.
15. Milk of magnesia is an aqueous suspension of magnesium hydroxide [Mg(OH)2] used to treat acid indigestion. Calculate the volume of a 0.035 M HCl solution (a typical acid concentration in an upset stomach) needed to react with two spoonfuls of milk of magnesia [approximately 10.0 mL at 0.080 g Mg(OH)2/mL]. (0.80 L)
16. A 1.00 g sample of a metal X (that is known to form X2+ ions) was added to a 0.100 L of 0.500 M H2SO4. After all the metal had reacted, the remaining acid required 0.0334 L of 0.500 M NaOH solution for neutralization. Calculate the molar mass of the metal and identify the element.
17. A 60.0 mL 0.513 M glucose (C6H12O6) solution is mixed with 120.0 mL of 2.33 M glucose solution. What is the concentration of the fnal solution? Assume the volumes are additive. (1.73 M)
18. Someone spilled concentrated sulfuric acid on the floor of a chemistry laboratory. To neutralize the acid, would it be preferable to pour concentrated sodium hydroxide solution or spray solid sodium bicarbonate over the acid? Explain your choice and the chemical basis for the action. (NaHCO3. NaOH is caustic and expensive)
19. These are common household compounds: table salt (NaCl), table sugar (sucrose), vinegar (contains acetic acid), baking soda (NaHCO3), washing soda (Na2CO3.10H2O), boric acid (H3BO3, used in eyewash), epsom salt (MgSO4.7H2O), sodium hydroxide (used in drain openers), ammonia, milk of magnesia [Mg(OH)2], and calcium carbonate. Based on what you have learned in this chapter, describe test(s) that would enable you to identify each of these compounds.
20. A 0.8870-g sample of a mixture of NaCl and KCl is dissolved in water, and the solution is then treated with an excess of AgNO3 to yield 1.913 g of AgCl. Calculate the percent by mass of each compound in the mixture. (44.11% NaCl; 55.89% KCl).
21. Phosphoric acid (H3PO4) is an important industrial chemical used in fertilizers, detergents, and in the food industry. It is produced by two different methods. In the electric furnace method elemental phosphorus (P4) is burned in air to form P4O10, which is then reacted with water to give H3PO4. In the wet process the mineral phosphate rock [Ca5(PO4)3F] is reacted with sulfuric acid to give H3PO4 (and HF and CaSO4). Write equations for these processes and classify each step as precipitation, acid-base, or redox reaction.
22. Give a chemical explanation for each of these: (a) When calcium metal is added to a sulfuric acid solution, hydrogen gas is generated. After a few minutes, the reaction slows down and eventually stops even though none of the reactants is used up. Explain. (b) In the activity series aluminum is above hydrogen, yet the metal appears to be unreactive toward steam and hydrochloric acid. Why? (c) Sodium and potassium lie above copper in the activity series. Explain why Cu2+ ions in a CuSO4 solution are not converted to metallic copper upon the addition of these metals. (d) A metal M reacts slowly with steam. There is no visible change when it is placed in a pale green iron(II) sulfate solution. Where should we place M in the activity series? ((a) The precipitate CaSO4 formed over Ca prevents the Ca from reacting with the sulfuric acid. (b) Aluminum is protected by a tenacious oxide layer (Al2O3). (c) These metals react more readily with water. (d) The metal should be placed below Fe and above H.)
23. A number of metals are involved in redox reactions in biological systems in which the oxidation state of the metals changes. Which of these metals are most likely to take part in such reactions: Na, K, Mg, Ca, Mn, Fe, Co, Cu, Zn? Explain.
24. The recommended procedure for preparing a very dilute solution is not to weigh out a very small mass or measure a very small volume of a stock solution. Instead, it is done by a series of dilutions. A sample of 0.8214 g of KMnO4 was dissolved in water and made up to the volume in a 500-mL volumetric flask. A 2.000-mL sample of this solution was transferred to a 1000-mL volumetric fl ask and diluted to the mark with water. Next, 10.00 mL of the diluted solution were transferred to a 250-mL fl ask and diluted to the mark with water. (a) Calculate the concentration (in molarity) of the fnal solution. (b) Calculate the mass of KMnO4 needed to directly prepare the fnal solution. ((a). 8.312 x 10-7 M (b) 3.284 x 10-5 g)
25. A 325-mL sample of solution contains 25.3 g of CaCl2. (a) Calculate the molar concentration of Cl- in this solution. (b) How many grams of Cl- are in 0.100 L of this solution?
26. Acetylsalicylic acid (C9H8O4) is a monoprotic acid commonly known as “aspirin.” A typical aspirin tablet, however, contains only a small amount of the acid. In an experiment to determine its composition, an aspirin tablet was crushed and dissolved in water. It took 12.25 mL of 0.1466 M NaOH to neutralize the solution. Calculate the number of grains of aspirin in the tablet. (One grain = 0.0648 g.) (4.99 grains)
27. This “cycle of copper” experiment is performed in some general chemistry laboratories. The series of reactions starts with copper and ends with metallic copper. The steps are: (1) A piece of copper wire of known mass is allowed to react with concentrated nitric acid [the products are copper(II) nitrate, nitrogen dioxide, and water]. (2) The copper(II) nitrate is treated with a sodium hydroxide solution to form copper(II) hydroxide precipitate. (3) On heating, copper(II) hydroxide decomposes to yield copper(II) oxide. (4) The copper(II) oxide is reacted with concentrated sulfuric acid to yield copper(II) sulfate. (5) Copper(II) sulfate is treated with an excess of zinc metal to form metallic copper. (6) The remaining zinc metal is removed by treatment with hydrochloric acid, and metallic copper is f ltered, dried, and weighed. (a) Write a balanced equation for each step and classify the reactions. (b) Assuming that a student started with 65.6 g of copper, calculate the theoretical yield at each step. (c) Considering the nature of the steps, comment on why it is possible to recover most of the copper used at the start.
28. Ammonium nitrate (NH4NO3) is one of the most important nitrogen-containing fertilizers. Its purity can be analyzed by titrating a solution of NH4NO3 with a standard NaOH solution. In one experiment a 0.2041-g sample of industrially prepared NH4NO3 required 24.42 mL of 0.1023 M NaOH for neutralization. (a) Write a net ionic equation for the reaction. (b) What is the percent purity of the sample? ((a) NH4+ + OH- → NH3 + H2O. (b) 97.99%.)
29. Hydrogen halides (HF, HCl, HBr, HI) are highly reactive compounds that have many industrial and laboratory uses. (a) In the laboratory, HF and HCl can be generated by reacting CaF2 and NaCl with concentrated sulfuric acid. Write appropriate equations for the reactions. (Hint: These are not redox reactions.) (b) Why is it that HBr and HI cannot be prepared similarly, that is, by reacting NaBr and NaI with concentrated sulfuric acid? (Hint: H2SO4 is a stronger oxidizing agent than both Br2 and I2.) (c) HBr can be prepared by reacting phosphorus tribromide (PBr3) with water. Write an equation for this reaction.
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