HOMEWORK #4

 1.    modified from Appelo and Postma (chapter 4)

A groundwater sample contains 17 ppm Ca²⁺ and 10 ppm HCO₃^-. Temperature is 25°C, pH is 9.9 (coincidentally the pH of Austin tap water) and you can neglect ionic strength.

a) Calculate the equilibrium concentration of CO₃^2- at this pH

b) Calculate SI for calcite

c)  Is the solution under-saturated, saturated or supersaturated with calcite.

2.   Calculate the concentration of As(V) species present in hydrothermal waters from El

 El Tatio. C_T,As(V) = 18.83 ppm, pH = 6.76, Temp = 81°C, ionic strength = 0.2.

     steps:

      a) Calculate the activity of As(V)

      b) Given the following equilibrium reaction, calculate a new Ka for 81°C

                              H2AsO4-  ßà HAsO42- + H+      log K25°C = -6.67          

c) Sketch a rough Bjerrum plot and estimate concentrations based on the 

graph. As(V) also forms AsO43- and H3AsO4 species (pK1 = 2.24, pK2 = 6.67, pK3 = 11.60), but at our pH they at can probably be neglected, why?

            d) Calculate the concentrations of the two dominant arsenate species.

3.      (S&J 4.2) Calculate the pH of the following potassium hydroxide, KOH, solutions:

a)      10^-4 M.

b)      10^-6 M.

4.      (S&J 4.6) A solution of 10^-2 mole of Na₂HPO₄ is added to 1 L of distilled water.

      a)   Write the pKa’s for the phosphate species and make a rough Bjerrum plot.

b)   Will the addition of Na₂HPO₄ bring the pH up or down?

c)      Write the mass balances for Na and PO₄, C_T,Na, and C_{T, PO4}.

d)      Write the proton condition, indicating the PRL, > PRL and < PRL species.

e)      Write the charge balance.

5.    (S&J 4.7) What pH results when 10^-2 mole of HF are added to 1 L of distilled water? 

       The temperature = 25 °C; assume ionic strength = 0.