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20.12 Copper is purified by electrolysis (see Figure 20.6). A 5.00-kg anode is used in a cell where
the current is 37.8 A. How long (in hours) must the current run to dissolve this anode and
electroplate it onto the cathode?
20.14 How would you obtain zinc from sphalerite (ZnS)?
20.16 A certain mine produces 2.0 X 108 kg of copper from chalcopyrite (CuFeS2) each year. The
ore contains only 0.80 percent Cu by mass. (a) If the density of the ore is 2.8 g/cm3, calculate
the volume (in cm3) of ore removed each year. (b) Calculate the mass (in kg) of SO2 produced
by roasting (assume chalcopyrite to be the only source of sulfur).
20.18 Although iron is only about two-thirds as abundant as aluminum in Earth’s crust, mass for
mass it costs only about one-quarter as much to produce. Why?
20.28 Write a balanced equation for each of the following reactions: (a) sodium reacts with water; (b)
an aqueous solution of NaOH reacts with CO2; (c) solid Na2CO3 reacts with a HCl solution; (d)
solid NaHCO3 reacts with a HCl solution; (e) solid NaHCO3 is heated; (f) solid Na2CO3 is
20.30 Calculate the volume of CO2 at 10.0°C and 746 mmHg pressure obtained by treating 25.0 g of
Na2CO3 with an excess of hydrochloric acid.
20.34 Starting with magnesium and concentrated nitric acid, describe how you would prepare
magnesium oxide. [Hint : First convert Mg to Mg(NO3)2. Next, MgO can be obtained by
heating Mg(NO3)2]
20.36 The second ionization energy of magnesium is only about twice as great as the first, but the
third ionization energy is 10 times as great. Why does it take so much more energy to remove
the third electron?
20.38 Helium contains the same number of electrons in its outer shell as do the alkaline earth
metals. Explain why helium is inert whereas the Group 2A metals are not.
20.40 Write chemical formulas for (a) quicklime, (b) slaked lime, (c) limewater.
20.44 With the Hall process, how many hours will it take to deposit 664 g of Al at a current of 32.6 A?
20.46 The overall reaction for the electrolytic production of aluminum by means of the Hall process
may be represented as
Al2O3(s) + 3C(s) → 2Al(l) + 3CO(g)
At 1000°C the standard free-energy change for this process is 594 kJ. (a) Calculate the
minimum voltage required to produce 1 mole of aluminum at this temperature. (b) If the actual
voltage applied is exactly three times the ideal value, calculate the energy required to
produce 1.00 kg of the metal.
20.48 Write a balanced equation for the thermal decomposition of aluminum nitrate to form
aluminium oxide, nitrogen dioxide, and oxygen gas.
20.50 The pressure of gaseous Al2Cl6 increases more rapidly with temperature than predicted by the
ideal gas equation even though Al2Cl6 behaves like an ideal gas. Explain.
20.52 Explain the change in bonding when Al2Cl6 dissociates to form AlCl3 in the gas phase.
20.54 When 1.164 g of a certain metal sulfide was roasted in air, 0.972 g of the metal oxide was
formed. If the oxidation number of the metal is +2, calculate the molar mass of the metal.
20.56 Referring to Figure 20.6, would you expect H2O and H+ to be reduced at the cathode and
H2O oxidized at the anode?
20.58 Given that ∆G°f(Fe2O3) = -741.0 kJ/mol and that ∆G°f(Al2O3) = -1576.4 kJ/mol, calculate ∆G°
For the following reactions at 25°C :
(a) 2Fe2O3(s) → 4Fe(s) + 3O2(g)
(b) 2Al2O3(s) → 4Al(s) + 3O2(g)
20.60 When an inert atmosphere is needed for a metallurgical process, nitrogen is frequently used.
However, in the reduction of TiCl4 by magnesium, helium is used. Explain why nitrogen is not
suitable for this process.
20.62 Explain each of the following statements: (a) An aqueous solution of AlCl3 is acidic. (b) Al(OH)3
is soluble in NaOH solutions but not in NH3 solution.
20.64 Write a balanced equation for the reaction between calcium oxide and dilute HCl solution.
20.66 Explain why most metals have a flickering appearance.
20.68 Describe a medicinal or health – related application for each of the following compounds: NaF,
Li2CO3, Mg(OH)2, CaCO3, BaSO4, Al(OH)2NaCO3 (see p.639).
20.70 Lithium and magnesium exhibit a diagonal relationship in some chemical properties. How does
lithium resemble magnesium in its reaction with oxygen and nitrogen? Consult a handbook of
chemistry and compare the solubilities of carbonates, fluorides, and phosphates of these