practice questions

Question 1

calculate the pH at which the activities of SO4(2-) and HSO4(-) are equal

Answer 1

-1.Form 2 equations for dissociation of H2SO4 and HSO4
- H2SO4 ⇌ H+ + HSO4− Ka1 = 1.0 × 10−2

HSO4− ⇌ H+ + SO42− Ka2 = 1.2 × 10−2

2. at pH where the activities of SO4 and HSO4 are equal, concentration of H+ ions will be the same
3. set IAP equal to eachother
   - [H+][HSO4-] = [H+][SO42-]
   - divide both sides by [H+]
4. Set up K equation for each reaction
   - Ka1 = [H+][HSO4-]/[H2SO4]
   Ka2 = [H+][SO42-]/[HSO4-]
5. substitute [HSO4-] = [SO42-] and solve for [H+]
   - [H+] = sqrt(Ka1 × Ka2/[H2SO4])
6. take -ve log of each side to get pH
   - pH = 0.5 × (pKa1 + pKa2) - 0.5 × log[H2SO4]
   where pKa1 = 2 and pKa2 = 1.92
7. pH = 0.5 × (2 + 1.92) - 0.5 × log[H2SO4]

pH = 1.96 - 0.5 × log[H2SO4] (Need to know concentration of H2SO4 to solve)
(assume its concentration is equal to the initial concentration of H+ ions if it is strong acid that completely dissociates in water)

8. Find [H+] by using equation for dissociation of water
   - Kw = [H+][OH-] = 1.0 × 10^-14
9. assume neutral conditions where [H+] = [OH-]
   - [H+] = [OH-] = sqrt(Kw) = 1.0 × 10^-7
10. Substitute into equation for pH and solve
    pH = 1.96 - 0.5 × log(1.0 × 10^-7)

pH = 1.96 + 3.5

pH = 5.46

Question 2

How would you measure the kinetics of barite dissolution?

Answer 2

Preparation of the solution: Prepare a solution with a known composition, pH, and temperature. The composition of the solution should be similar to the conditions of the natural environment where barite dissolution occurs. The pH and temperature should be kept constant throughout the experiment.

Weighing of the barite: Weigh a known amount of barite and add it to the solution.

Sampling: At regular intervals, remove a small amount of solution and analyze it for barium and sulfate concentrations.

Analysis of the solution: Analyze the solution for barium and sulfate concentrations using ICP or ion chromatography. The change in barite concentration can be calculated by comparing the initial and final concentrations of barium and sulfate.

Calculation of dissolution rate: The dissolution rate can be calculated by dividing the change in barite concentration by the duration

Question 3

How does barite affect the concentration of Ra in the soil?

Answer 3

1. Write out chemical reaction for dissolution of Barite
   BaSO4-> Ba + SO4
2. Calculate the change in Gibbs free energy associated with the dissolution of BaSO4
   - find delta Gf of products and reactants and subtract reactants from products
   - ∆G = ∆G° + RT ln(K)
   - find K

Question 4

List chemical elements that can substitute for Fe and MG in the octahedral layer of smectite. Constrain yourself to rows 3 and 4 of the periodic table