Geology 428/628 Geochemistry
Sample Questions for Exam 1

1. Sketch a log activity vs. pH diagram for "arsenate speciation" resulting from the dissolution of 0.01 M of H3AsO4 in water.
Show how you determined the concentration and pH of one of the adjacent species crossing points.

2. A wetland water at 25 degrees C has activities of 734 mg/L SO4-2 and 427 mg/L Ca2+.
a) What are these activities as expressed in mol/L?
b) Calculate I for this simple water (for now we are ignoring all other chemical constituents; assume density = 1).
c) Use the Davies equation to calculate an initial activity coefficient for SO4-2 and Ca2+.

3. Describe the components that make up these typical rock types:
a) shale b) sandstone c) carbonates

4. What are the activities of Cu2+ and SO4-2, and the pH of a pure water (closed to atmosphere) in equilibrium with bronchantite (Cu4(OH)6SO4)?

5. Calculate the solubility of barite (BaSO4) in water at 25 deg. C, assuming ideal behavior, expressed as:
a. ppm Ba
b. ppm SO4-2
c. ppm SO4-2 if 0.001 M SO4-2 had already been in solution previously.

6. Cobalt hydroxide, Co(OH)2, is also known as the mineral transvaalite. Write equations for the dissociation of Co(OH)2 using K1, K2, KA, and KAq in Faure, for a CO2 -free system. Derive relationships for log activities vs. pH for each species. Calculate the total species activity at pH = 10.


1. Data from Faure, Table 9.3, p. 120.: pK1=2.2; pK2=7.0; pK3=11.5;

Calculation of crossing point of H3AsO4 with H2AsO4-1:

10-2.2 = [H+]*[H2AsO4-1] / [H3AsO4]

at crossing, [H2AsO4-1] / [H3AsO4] = 1.0

So, 10-2.2 = [H+] and pH = 2.2.

For concentration, remember the total activity is 10-2.0M = sum of all As species in solution.

At pH - 2.2, [HAsO4-2] and [AsO4-3] will be negligible, so we can estimate that [H3AsO4] + [H2AsO4-1] = approx. 10-2.0.

Also, at the crossover, [H3AsO4] = [H2AsO4-1].

By substituting, we will find that [H3AsO4] = [H2AsO4-1] = 0.5*(10-2.0) = 10-2.3.

7.64 x 10-3 or 10-2.12M SO4-2
1.07 x 10-2 or 10-1.97M Ca+2

I = 3.67 x 10-2 = 0.037

Using the Davies equation (assume 25 deg. C), gamma for both ions = 0.486 (compare with Table 10.5)

Questions 3-6: Similar to homeworks or exercises done in class.

Here is a homework with a solution from Dr. Alba Torrents of University of Maryland.

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