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PLS 671: Environmental Soil Chemistry
 

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Problem Set #1
“Review of Chemical Principles and Clay Minerals”

Due: September 16, 2004





1.) a.) Place the following atoms in the order of increasing size:  Br, F, Cl, Cs.

b.) Place the following atoms in the order of decreasing electronegativity:   F, O, N, C
 

2.)
A recipe for preparing a soil extractant solution specifies that you add 1.0 g of sodium dithionite (Na2S2O 4) to 200 mL of a solution of 0.2 M sodium citrate.  Assuming that the final volume remains 200 mL, what is the final molar concentration of citrate, dithionite, and sodium in this solution?  
(Atomic weight:  Na, 22.9g mol-1; O, 16.0 g mol-1; S, 32.1 g mol-1)
 

3.)
Find the pH value of the following solution at 25 oC: 0.01 M sodium acetic acid if the pKa is 4.5.  Show the number of species in solution, the total number of independent equations needed to solve for these unknowns, and the approximation equations used.
 

4.)
Given below the following pairs of cations and anions, determine the radius sum in Å, radius ratio, coordination
number (CN), and geometrical (crystal) arrangement for each pair.  Use the ionic radii values presented in
Tables 2.3 of the text book ..

a.) Fe3+ and Cl-


b.) Na+ and Cl-


c.) Ca2+ and Cl-


d.) Si4+ and Cl-

 

5.)
You analyze the <2μm clay fraction of a soil from Kentucky using x-ray diffraction.  The d-spacings (001)
are given below.  Identify the clay mineral suite and provide a rationale for the identification.

K-saturation
         Mg-saturation

T(K)                      d-spacings (nm)
T(K)                d-spacings (nm)

              298                        0.72, 1.0, 1.4
298                  0.717, 1.0, 1.77

              383                        0.73, 1.0, 1.38
383                  0.72, 1.0, 1.4

              573                         0.72, 1.0, 1.4


              823                         1.0




6.)

One major mechanism of N loss from soils is denitrification.  Fit the data below using the best-fit elementary
rate equation and estimate the rate coefficient and the time needed for 1/8 of the initial concentration of
NO3- to be remaining.  Would the t1/2 be affected if the initial 
NO3- concentration was doubled?


Time (days)
NO3- (mg L-1)

0
240

5
167

10
125

25
50

40
20