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 pK_a1 4.58 and pK_a2 9.46. Answer the following questions.

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

pK_a1 4.58 due to COOH and pK_a2 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