PLS 671                                                                      Soil Chemistry

Problem Set #2                                                           Due: October 3, 2006

“Clay Minerals and Soil Organic Matter”               KEY


1.  (20) A 1 M Na+ salt solution is added to 2 g of air-dried soil (10% gravimetric water content) to measure CEC.  The Na+-saturated soil is treated with sequential washings of a 1 M NH4+ salt and the washings are collected in a 50 mL volumetric flask and filled to volume with deionized water.  The solution in the 50 mL volumetric flask is analyzed to contain a total Na concentration of 200 mg L-1.  Calculate the CEC in cmol(+) kg-1 on an oven-dried weight basis.   



2.  (20) The soil in #1 above is fractionated and found to contain 30% clay.  What mineral probably dominates the mineralogy of the soil? 


  Montmorillonitic or vermiculitic


3.  (40)  A diprotic organic acid, p-hydroxybenzoic acid, has been identified in soil solutions.  The two acidity constants are pKa1 4.58 and pKa2 9.46.  Answer the following questions.

            a.  Which functional groups correspond to the acidity constants above?


pKa1 4.58 due to COOH and pKa2 9.46 due to phenolic OH



            b.  For a total p-hydroxybenzoic acid concentration of 2 × 10-3 M, derive equations that define the concentration of each species versus pH.  Plot the species concentration versus pH. 





4.  (20) Calculate the relative molecular mass of a fulvic acid molecule with the chemical formula C130H180O93N5S. 


Relative molecular mass= 3333