Third Homework

1.The ideal gas law, PV=nRT, can be applied to the constituents of the gaseous phase in soils to a good approximation.

H = [A(g)]/[A(aq)]

where [ ] is the concentration of species A, , and H = 103/KHRT is a dimensionless constant based on R= 0.08205 atmLmol-1K-1, and T in degrees Kelvin.

A.Complete the following table, based on the equation above for H.

Gas / KH
(molm-3atm-1) / H / Gas / KH
(molm-3atm-1) / H
CO2 / 34.06 / NO / 1.88
CH4 / 1.50 / O2 / 1.26
NH3 / 5.76x104 / SO2 / 1.24x103
N2O / 25.55 / H2S / 1.02x102

2.Gypsum has been suggested as a useful amendment in oxisols (highly weathered soils) and soils impacted by acid rain to control aluminum toxicity to plants. One possible reaction underlying the beneficial effect of gypsum is:

Al(OH)3(s,am) + CaSO4.2H2O(s) + H2O(l) = AlOHSO4.5H2O(s)+ Ca(OH)2(s)

where AlOHSO4(s).5H2O is the mineral jurbanite. Given the following data

° [Al(OH)3] = -1139.8 kJ mol-1

° [CaSO4.2H2O] = -1797.2 kJ mol-1

° [H2O] = -237.13 kJ mol-1

° [Ca(OH)2] = -898.48 kJ mol-1

° [AlOHSO4.5H2O] = -2679.1 kJ mol-1

A.Balance the reaction.

B.Is the reaction thermodynamically favorable?

C.What is the log K for this reaction assuming that the components are in their standard states?

3. The following data were collected for the chemical analyses of four soil solutions after 10-6 M CuSO4 was added

3. A metal fabrication plant is discharging wastes containing Pb into the ocean. Because seawater contains high concentrations of sulfate and chloride, they believe that the aqueous phase lead activities in seawater will be controlled by the solubility of PbSO4 (anglesite) which has a solubility product of Ksp = 10-7.79.

A.Use a speciation model to predict the changes in Pb speciation as the lead is transported from the waste stream (fresh water) to seawater near the effluent pipe. Assume the following conditions. List any assumptions you made [Hint: there is not enough data given in the problem, so you have to decide how to set up a reasonable problem].

•Temperature is 25C.

•pH of the seawater is 8.1.

•The total concentrations (mol/kg) in seawater of chloride and sulfate are Cl- = 0.56 and SO42- = 0.028, respectively.

B.What is (are) a possible reason(s) to explain differences between model predictions and observations if you collected seawater and tested for lead, and found that the total concentration of lead in the water exceeded the predictions made by the geochemical speciation program.

4. Quantitative structure activity relationships (QSAR) are linear regressions between a molecular property and the structure of organic compounds. Estimating the property of one molecule based on the properties of an analogous series of compounds ensures that the molecular interactions between these molecules and water are fairly consistent. It is these interactions which control solubility.

A.Make a plot of the log solubility (log Cwsat)-molar volume data of an analogous series of organic compounds. Use linear regression to quantify this relationship. Include the best fit line on your graph.

B.Estimate the aqueous solubility of hexachloroethane (HCE) at 25C using the relationship (as estimated by linear regression) and graph created above.

Compound / mw (g/mol) / 25 (g/ml) / Vorg (mL/mol) / log Cwsat (l,L) (mol/L)
chloromethane (L) / 50.5 / 0.93 / 54.5 / -0.22
dichloromethane / 54.9784.97 / 1.317 / 64.5 / -0.64
trichloromethane / 119.4 / 1.4 / 85 / -1.19
tetrachloromethane / 153.8 / 1.5 / 100 / -2.20
1,1 dichloroethane / 99.0 / 1.168 / 84.5 / -1.30
1,2 dichloroethane / 99.0 / 1.2 / 85 / -1.07
1,1,1 trichloroethane / 133.4 / 1.3 / 110 / -2.07
1,1,2,2, tetrachloroethane / 167.9 / 1.587 / 105.8 / -1.74
 1,2,3,4,5,6 hexachlrorocyclohexane / 290.8 / 1.5 / 200 / -4.90
hexachlroroethane

5.Consider a near-surface aquifer contaminated with lead. In this problem assume all activities equal concentrations (i.e.,  = 1)

A. Construct a log concentration (M) versus pH graph for the reactions of lead in the aquifer given the following equations:

PbO(s) + 2H+ = Pb2+(aq) + H2Olog cKso= 12.7

PbO(s) + H+ = PbOH+(aq)log cKs1= 5.0

PbO(s) + H2O = Pb(OH)2o(aq)log cKs2= -4.4

PbO(s) + 2H2O = Pb(OH)3-(aq) + H+log cKs3= -15.4

B. Given the diagram that you constructed in 3A, at what pH would solid PbO precipitate given a total [PbT] = 1x10-5 M

C. Assuming that the solid PbO and liquid water are in their standard states, and that all activity coefficients are equal to one, what would be the equilibrium pH if PbO(s) was placed in pure water? (Hint: construct an equation for electroneutrality in the solution and solve it).

6. The Kesterson reservoir in the Central Valley of California has a selenium problem. Agricultural water leaching through soil and groundwater has leached Se from the geologic matrix. The Se is present in the soil and sediments because this material originated from marine sediments. Selenium can exist in several redox forms including SE(VI) [SeO42-, HSeO4-] and Se(VI) [SeO32-, HSeO3-]. The Se(VI) form of selenium exhibits much higher toxicity then the SE(IV) form.

Use the following redox reaction for Se and Minteqa2 to graph the change in activity ratio of Se(VI)/SE(IV) as a function of pE. (Hint: Specify a redox couple between SeO42- and HSeO3- in Edit Level II, number 5 (follow directions in manual)). Use the following conditions for the other ions present:

SeO42-(aq) + 3H+(aq) + 2e(aq) = HSeO3-(aq)log KR = 36.319

pH fixed at 8.20, pE fixed at -4, -2, 0, 2, 4, 6, 8

Ion / pConc.
Ca2+ / 2.70
Mg2+ / 3.00
Na+ / 2.43
K+ / 3.10
SO42- / 2.00
Cl- / 2.00
SeO42- / 1.70

B. How sensitive is the redox reaction given in part A to changes in pH? Graph the distribution (%) of SeO42 and HSeO3 as a function of pE at several different pH’s that bracket natural conditions. The Y axis should be %, the X axis is pE. Use lines of several colors or symbols to represent the different pH’s.

7. In a large number of fish toxicity studies, it has been found for various organic compound class that acute toxicity correlates with their octanol-water partition coefficients (Kow), where the Kow describes the relative concentration distribution of an organic between water and octanol, an immisicble organic solvent:

Using LC50 (the concentration at which half of the organisms die) as a measure of toxicity, it has been found that toxicity is related to log Kow with an expression of the general form:

A. Draw the structures for 1,2,4 trichlorobenzene (TCB) [log Kow = 4.00; LC50 = 1.3e-5 M] and 2,4-dinitrotoluene (DNT) [log Kow = 1.98; LC50 = 6.9e-5 M] .

B. Using the data given below for a number of substituted benzenes, estimate the LC50 for these 2 compounds and compare to the observed LC50. Think about whether you want to make one regression using all of the data below or to divide the data into two groups (to make 2 regression lines) based on organic structures. Provide the values for your regression coefficients. Discuss your findings.

Compound / LC50 (umol L-1) / Log Kow
Toluene / 740 / 2.69
1,3-dimethylbenzene / 355 / 3.15
chlorobenzene / 170 / 2.92
1,2 dichlorobenzene / 40 / 3.38
1,4 dichlorobenzene / 27 / 3.38
1,2,3,4 tetrachlorobenzene / 3.7 / 4.55
1,23,5 tetrachlorobenzene / 3.7 / 4.65
Pantachlorobenzene / 0.70 / 5.03
nitrobenzene / 500 / 1.83
2-nitrotoluene / 240 / 2.30
4-nitrotoluene / 270 / 2.39
2-chloronitrobenzene / 190 / 2.26
4-chloronitrobenzene / 38 / 2.35
3,5-dichloronitrobenzene / 30 / 3.13
1,2-dinitrobenzene / 7 / 1.58
1,4 Dinitrobenzene / 2.3 / 1.47

1