List the following substances in order of energy released by combustion per gram: C$_8$H$_{18}$, H$_2$, C(s), CH$_4$. (See Study Question 22 for the enthalpy of combustion of isooctane, C$_8$H$_{18}$.) Which is the best fuel based on a comparison of kJ/g of fuel?

Energy consumption in the United States amounts to the equivalent of the energy obtained by burning 7.0 gal of oil or 70. lb of coal per day per person. Using data in Table 20.4, carry out calculations to show that the energy evolved from these quantities of oil and coal is approximately equivalent. The density of fuel oil is approximately 0.8 g/mL. (1.00 gal = 3.785 L and 1.00 lb = 454 g)

Assume the average $\mathrm{CO}_2$ concentration in the atmosphere is $397 \mathrm{ppm}$ (as of February 2013). The actual concentration of $\mathrm{CO}_2$ at different sites will vary. Speculate on whether the concentration of $\mathrm{CO}_2$ would be expected to be higher, lower, or the same as this average value in a typical large city.

In the discussion on the composition of air, mention is made of the fact that water vapor may have a concentration as high as 40,000 ppm. Calculate the partial pressure exerted by water vapor at this concentration. Assume that this represents a situation with 100% humidity. What temperature would be needed to achieve this value?

The enthalpy of combustion of isooctane (C$_8$H$_{18}$), one of the many hydrocarbons in gasoline, is 5.45 $\times$ 10$^3$ kJ/mol. Calculate the enthalpy change per gram of isooctane and per liter of isooctane (d = 0.688 g/mL).

Per capita energy consumption in the United States has been equated to the energy obtained by burning 70. lb of coal per day. Use enthalpy of formation data to calculate the energy transferred as heat, in kilojoules, when 70. lb of coal is burned. (Assume the enthalpy of combustion of coal is 33 kJ/g.)

Hydrogen can be produced using the reaction of steam (H$_2$O) with various hydrocarbons. Compare the mass of H$_2$ expected from the reaction of steam with 100.g each of methane, petroleum, and coal. (Assume complete reaction in each case. Use CH$_2$ and C as the representative formulas for petroleum and coal, respectively.)

Silver ions have long been known to have biocidal properties. The increasing use of silver for this purpose has led to greater silver concentrations in waste water, and there has been several concern regarding it. However, recent studies revealed that the silver was largely put back into the environment as ${Ag}_2 {~S}$. The solubility of this sulfide is $1.4 \times 10^{-4} {~g} / {L}$.

(a) Calculate the $K_{\text {sp }}^{\prime}$ for ${Ag}_2 {~S}$.

(b) How does the solubility of ${Ag}_2 {~S}$ change as the pH decreases?

A 2010 article in Science magazine described "Sewage Treatment with Anammox." The authors noted that "Fixed nitrogen such as ammonium and nitrate [ions] should be removed [from sewage] to avoid toxic algal blooms in the environment." One approach to removing ammonium ions is to use ammonium-oxidizing (anammox) bacteria. Two (unbalanced) reactions that are involved are:

$$\begin{aligned} & {NH}_4{ }^{+}({aq})+{NO}_2{ }^{-}({aq}) \rightarrow {N}_2({~g})+{H}_2 {O}(\ell) \\ & {NH}_4{ }^{+}({aq})+{O}_2({~g}) \rightarrow {NO}_2{ }^{-}({aq})+{H}_2 {O}(\ell) \end{aligned}$$

Balance these two equations in acid solution.

Calculate the mass of Mg that could be obtained by the process described in previous study question from 1.0 L of seawater. To prepare 100. kg of Mg, what volume of seawater would be needed? (The density of seawater is 1.025 g/cm$^3$.)

Although there are a number of magnesium containing minerals, the commercial source of this metal is seawater. Treatment of seawater with Ca(OH)$_2$ gives insoluble Mg(OH)$_2$. This reacts with HCl to produce MgCl$_2$, which is dried, and the metal is obtained by electrolysis of the molten salt. Write balanced net ionic equations for the three reactions described here.

In any solution, there must be a balance between the positive and negative charges of the ions that are present. Use the data in Table 20.2 to determine how close to a balance of charge is achieved for the ions in seawater. (The table lists only the major ions in seawater, and the balance between positive and negative charge will only be approximate.)

Match the substances on the left with the basic units that compose them on the right.

$$\begin{array}{ll} \mathrm{NI}_3 & \text { molecules } \\ \text { copper metal } & \text { single atoms } \\ \mathrm{SrCl}_2 & \text { diatomic molecules } \\ \text { nitrogen } & \text { formula units } \end{array}$$

State which of the alcohols listed in above Problem you would expect to be the best solvent for each of the following substances, and explain why: (a) $\mathrm{I}_2$, (b) KBr, (c) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3$.