QUESTION 1 [4 marks]
A lake water is found to have the following characteristic:
pH = 6.0
CT = 1.0 × 10-3
Cu(II)T = 1.0 × 10-6
Assume no gas exchange with the atmosphere and the soluble copper(II) consists only of Cu2+ and CuOH+
.
Consider only the solid Cu(OH)2(s) and Cu2(OH)2(CO3)(s) (malachite).
a) Determine which solid, if any, controls the solubility of copper(II) in the lake. (1)
b) A lake liming project in an adjacent watershed raises the pH of the lake to 7.0. Which solid, if any,
now controls the solubility of copper(II) in the lake? What is the total soluble copper now? (2)
c) If the pH of the lake is raised to 8.0, which solid, if any, controls the solubility of copper(II) in the
lake? What is the total soluble copper? (1)
The pertinent equilibria are:
Cu(OH)2(s) = Cu2+ + 2OH– Ks0,1 = 10–19.32
Cu2(OH)2(CO3)(s) = 2Cu2+ + 2OH–
- CO3
2– Ks0,2 = 10–33.3
Cu2+ + OH–
= CuOH+ K = 10+6.5
QUESTION 2 [5 marks]
Consider a water with the following composition
pH = 7.8 Alk = 1.3 × 10-3
eq/L
Cd(II)T = 1.0 × 10-9 M CaT = 1.0 × 10-3 M
2
a) Is there a possibility that CdCO3(s) or Cd(OH)2(s) precipitate? (1)
b) Determine the speciation of Cd(II) in the water? (2)
c) How does the speciation of Cd(II) change when 10-7 mol EDTA is added to 1L of this water? (2)
The pertinent equilibria are: logK
Cd2+ + H2O = CdOH+ - H+
-10.1
Cd2+ + CO3
2-
= CdCO3
0
4.5
CdCO3(s) = Cd2+ + CO3
2-
-13.7
Cd(OH)2(s) = Cd2+ + 2OH-
-14.3
Cd2+ + EDTA4-
= CdEDTA2-
16.5
Ca2+ + EDTA4-
= CaEDTA2-
10.7
HEDTA3-
= EDTA4- - H+
-10.2
QUESTION 3 [6 marks]
a) An anoxic freshwater is characterized by: (3)
NaT = CaT = MgT = KT = ClT = [SO4] T = CT = [S(-II)] T = 10-3 M
i) Given pH = 8.2, calculate alkalinity, the carbonate, and the sulfide species and the partial
pressures of CO2(g) and H2S(g). (1)
(Note: CT = [H2CO3
*
] + [HCO3
] + [CO3
2-
] and [S(-II)] T = [H2S] + [HS-
] + [S2-
])
ii) Calculate the pe and the partial pressure of O2. (1)
iii) Is methane fermentation likely to take place in this system? (0.5)
iv) Write the complete redox reactions for methane fermentation of methanol (CH3OH) and of
formaldehyde (CH2O). (0.5)
What are the qualitative effects of such fermentation on alkalinity and pH? (bonus 1 mark)
b) Estimate pe for the aquatic systems characterized by the following information: (3)
i) Water from deeper layers of a lake having [O2(aq)] = 0.02 mg/L and a pH of 7.0. (1)
ii) An anaerobic digester in which 60% CH4(g) and 40% CO2(g) are in contact with water of pH
7.0. (1)
iii) A water sample of pH = 6.0 containing [SO4
2-
] = 10-3 M and smelling of H2S(g). It is
reasonable to set PH2S between 10-2
and 10-8
atm.
Sample Solution
