Hw4 Assignment Page 2
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2-2
pH 5.1
pH 8.1
pH 10.0
-3
-4
s=1
s=−1
-5
s=2
-6
-7
-8
2
3
4
5
6
7
8
9
10
11
12
pH
4. Acid drainage from coal mines can make the receiving waters uninhabitable by fish. To
remediate one such situation, a consultant has recommended dissolving limestone, CaCO
, in the
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drainage water to neutralize the acid. The pH of the untreated water is 2.40 due to the following
overall reaction:
( )
( )
(
) ( )
−
+
+
+
→
+
+
2
FeS
s
2.25 O
aq
2.5 H O
Fe OH
s
SO
2H
2
2
4
3
a. Determine is the concentration of TOTSO
in the drainage water using manual
4
−
calculations? Use the Davies equation to model activity coefficients. Is HSO
a strong
4
acid in this solution? (Hint: write out the mathematical expressions that relate the
−
+
2−
concentrations of H
, HSO
, and SO
to known values and/or each other. Some of these
4
4
relationships will include activity coefficients. Then, make a guess for the ionic strength of
the solution, compute the activity coefficients, and solve for the species concentrations.
Finally, test whether the ionic strength based on those concentrations is consistent with
your guess. If not, iterate on the guess until it converges with the computed value.)
b. What is the concentration of Ca
2+
in the stream after the solution has been neutralized to
pH 7.0? Assume that Fe(OH)
(s) is non-reactive after it precipitates.
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