CE 420/520 Environmental Engineering Chemistry
Problem Set No. 1 – Equilibria
Due Date: Sept. 15
1.
For the water analysis of Colorado River below (adapted from Benjamin, 2002):
(a)
Express the concentration of Ca2+ in terms of moles/liter and mg/L as CaCO3
(b)
Assuming that Na+ is the only significant species missing in the analysis, estimate its value by
using the principle of electroneutrality
(c)
Compute the activities of Ca2+, SO42- and Cl- using the Davies equation
Concentrations of major dissolved constituents in Colorado River
Ca2+
mg/L
77
Mg2+
mg/L
29
Na+
mg/L
?
Clmg/L
88
SO42- mg/L
250
HCO3- mg/L as CaCO3
135
DOC
mg/L
3
pH
8.3
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2.
3.
4.
5.
6.
7.
Problem 3, page 83
Problem 5, page 83
Problem 6, page 83
Problem 10, page 84
Problem 12, page 84
Problem 16, page 84
Extra Question for CE 520 Students
8.
The following "total analysis" of a wastewater is given below. Note that the pH is not given (adapted from
Benjamin, 2002).
(i)
Do a charge balance analysis to see whether the solution is electrically neutral?
(ii)
If H+ and OH- are the only ions missing from the analysis, what must be the values of their
activities be given the Kw for the dissociation of water is 10-14 and that the activity of H20 is
approximately 1.0. Assume activity coefficients can be modeled using the Guntelberg
approximation.
Note that Kw = 10-14 = {H+}{OH-}
(hint: since it is an equilibrium constant, you must start with the activities not with concentrations
in your computation - state assumptions clearly)
(iii)
Assuming that your answer for (ii) is correct, consider the reaction
Ca2+ + HCO3- < == > CaCO3 (s) + H+
and determine whether CaCO3 is over or undersaturated in the water at 25o C. Use the
thermodynamic data given below.
Species
 Gfo (kcal/mole)
Hfo (kcal/mole)
2+
Ca
-132.2
-129.8
HCO3-140.31
-165.18
H+
0
0
CaCO3 (s)
-269.7
-288.3
Water Analysis
NH3
NO2NO3FClHCO3SO42-
0.08 mg/L as N
0.008 mg/L as N
2.0 mg/L as N
21.1 mg/L as F
24.1 mg/L as Cl15 mg/L as C
20 mg/L as SO4
Na+
K+
Ca2+
227 mg/L as Na
18.3 mg/L as K
1.7 mg/L as CaCO3