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3.17 For the endothermic reaction, SO2(g) = S(s) + O2 will an increase in temperature increase,
decrease, or have no effect on the reaction’s equilibrium constant?
Solution:
3.18 What pH is required to reduce a high concentration of a dissolved Mg2+ to 25 mg/L? The
solubility product for the following reaction is 10-11.16.
Solution:
3.19 (a) What is the solubility (in moles/L) of CaF2 in pure water at 25°C?
(b) What is the solubility of CaF2 if the temperature is raised 10°C?
(c) Does the solubility of CaF2 increase, decrease, or remain the same if the ionic strength is
raised? (Explain your answer.)
Solution:
3.20 At a wastewater-treatment plant FeCl3(s) is added to remove excess phosphate from the
effluent. Assume that the reactions that occur are
FeCl3(s) Fe3+ + 3Cl-
Fe3+ + PO43- FePO4(s)
The equilibrium constant for the second reaction is 1026.4. What concentration of Fe3+ would be
needed to maintain the phosphate concentration below the limit of 1 mg P/L?
Solution:
3.21 One method to remove metals from water is to raise the pH and cause them to precipitate
as their metal hydroxides. (a) For the following reaction, compute the standard free energy of
reaction:
Cd2+ + 2OH- Cd(OH)2(s)
(b) The pH of water was initially 6.8 and was then raised to 8.0. Is the dissolved cadmium
concentration reduced to below 100 mg/L at the final pH? Assume the temperature of the water
is 25°C.
Solution:
3.22 Naphthalene has a log Kow of 3.33. Estimate its soil-water partition coefficient
normalized to organic carbon and also the 95% confidence interval of your estimate.
Solution:
3.23 Atrazine is an herbicide widely used for corn and is a common groundwater pollutant in
the corn-producing regions of the United States. The log Kow for atrazine is 2.65. Calculate the
fraction of total atrazine that will be adsorbed to the soil given that the soil has an organic carbon
content of 2.5%. The bulk density of the soil is 1.25 g/cm3; this means that each cubic
centimeter of soil (soil plus water) contains 1.25 g of soil particles. The porosity of the soil is 0.4.
Solution:
3
33
0.4
0.4 5.80
cm
cm cm
+
1.25 ng/L dissolved in the Bay water, and 250 ng/gm dry weight of sediment. Using the
information provided and assuming equilibrium, what is the sediment-water partition coefficient
for mercury in the sediments (units of cm3 per gram dry weight of sediment)?
Solution:
3.25 Given the following general reaction:
A + 2B + 3C P + 4Q
Show how the change in concentration of C with time is related to the change in concentration of
A, B, P, and Q with time.
Solution:
3.26 Which of the following statements about the study of chemical kinetics is true ? (a)
temperature has no effect on the rate of a reaction, (b) changes in reactant concentration do not
affect the rate at which a reaction occurs, (c) the addition of a catalyst to a reaction will speed up
the reaction but it will not ultimately result in a larger mass of product, (d) for the same reactants,
the larger the surface area, the slower a reaction will occur (problem from EPA Air Pollution
Training Institute, http://www.epa.gov/apti/bces/).
Solution:
3.27 Peridisulfate (S2O82-) reacts with thiosulfate (S2O32-) according to the following reaction:
S2O82- + 2 S2O32- 2SO42- + S4O62-
a) Show how the change in perdisulfate concentration with time is related to the change in
concentration with time of the other three species. b) If the reaction is elementary and
irreversible, what is the overall order of the reaction?
Solution:
3.28 A first-order reaction that results in the destruction of a pollutant has a rate constant of
0.l/day. (a) How many days will it take for 90% of the chemical to be destroyed? (b) How long
will it take for 99% of the chemical to be destroyed? (c) How long will it take for 99.9% of the
chemical to be destroyed?
Solution:
3.29 A strain of bacteria has been isolated that can cometabolize tetrachloroethane (TCA).
This strain of bacteria can be used for the bioremediation of hazardous-waste sites contaminated
with TCA. Assume that the biodegradation rate is independent of TCA concentration (i.e., the
reaction is zero order). In a bioreactor it is observed that the rate for TCA removal was 1 µg/L-
min. What water retention time would be required to reduce the concentration from 1 mg/L in
the influent to 1 µg/L in the effluent of a reactor? Assume the reactor is completely mixed.
Solution:
3.30 Assume that PO43- is removed from municipal wastewater through precipitation with Fe3+
according to the following reaction: PO43- + Fe3+ FePO4(s).
The rate law for this reaction is
3
433
4
PO Fe PO
dk
dt
−
+−
= −
(a) What is the reaction order with respect to PO43-?
(b) What order is this reaction overall?
Solution:
3.31 Obtain the World Health Organization (WHO) report on “Urine diversion: Hygienic risks
and microbial guidelines for reuse.” Review Figure 2 in Chapter 1 (Introduction) of the WHO
report. (a) How many grams of N, P, and K are excreted every day in a Swedish person’s urine?
Solution:
