EECE 505 Aquatic Chemistry Problem Set 5
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EECE 505 Aquatic Chemistry
Problem Set 5
Total points: 115 points (100 + 15)
Point values are shown in brackets. Show your work. Explain any assumptions that you make. You are encouraged to discuss the course material outside of class and are permitted to work together on problem sets; however, you are expected to turn in your own work. Unless explicitly asked to calculate activity coefficients, assume that the activity coefficients for solutes are one. You can make calculations in MINEQL and MINTEQ that 1) ignore ionic strength corrections to activity and 2) involve turning all solids off. If you use MINEQL to assist in solving a problem, you still must show work illustrating your calculations.
** Make sure that you submit Homework#4 (4) [15 point] along with Homework#5.
1. [25] A solution at pH 7.5 contains Cu2+ and NH3 .
a) What is the ratio {CuOH+}/{Cu2+}?
b) What is the ratio {NH4+}/{NH3}?
c) Are there any hydroxo complexes of Cu present at concentrations greater than that of CuOH+?
d) If {NH4+} = 10-3.0 , which Cu(NH3)x2+ complex will be present at the greatest concentration?
e) For a system with TOTCu = 10-3 M and at pH 7.5, what are TOTNH3 and {Cu2+} when Cu(NH3)42+ first becomes the dominant Cu species?
2. [15] Prepare a log C-pH diagram for an aqueous solution containing 10-6 M TOTZn and 10-4 M TOTNH3 , over the pH range from 4 to 10. What is the equilibrium partial pressure of NH3(g) in the system at pH 8.5?
3. [15] Determine the speciation of Cu, Zn, and a ligand L in a solution at pH 6.0, if the solution contains 10-6 M each of TOTZn, TOTCu, and TOTL, and if L- participates in the following reactions:
Cu2+ + HL ↔ CuL+ + H+ *K1 = 106.0
Zn2+ + L- ↔ ZnL+
HL ↔ H+ + L-
K1 = 107.0
Ka = 10-3.0
4. [20] A log C – pH diagram is provided below for a system containing 10-4 M total ferrous iron [ TOTFe(II)], 1.5 × 10-5 M total sulfide, and 2 × 10-4 M total CO3 . For all parts of the question, assume ideal solution behavior.
(a) If the solution is at pH 8.0, what is the total contribution (mol/L) of the six Fe- containing species to TOTH, if FeOH+ , HS- , and CO32- are chosen as components?
(b) What is the total contribution of the six Fe-containing species to the alkalinity of the solution, in equiv/L.
(c) Using only the information provided in the plot, determine *β3 for formation of Fe-OH complexes?
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
-8.5
-9.0
7
pH
5. [25] The diagram below (from Freeze and Cherry) shows the evolution of groundwater composition due to equilibration with calcite under two conditions: open-system and closed-system dissolution. For the closed-system dissolution, the initial composition of the infiltrating fluid is set by the equilibrium with CO2(g) at a fixed partial pressure but no further gas exchange is considered as the reaction with calcite progresses. The questions below are for a closed system.
a) Calculate the pH and CT of the infiltrating fluid before any reaction with calcite for PCO2 = 10-2 bar (i.e., “initial” composition).
b) Calculate the equilibrium (i.e., “final” composition) which results from the equilibration of the infiltrating fluid with calcite. You can use Ksp, calcite = 10-8.3 = [Ca2+][CO32-]
c) Proceeding step-wise from the “initial” values of pH and [HCO3-] to their “final” values, show that groundwater composition evolves along the curved path shown in the figure.
Source: Freeze and Cherry (1979), Groundwater
2022-10-25