CE 420/520 Environmental Engineering Chemistry
Problem Set No. 4
Dr. S.K. Ong
Due Date: Oct. 21, 2004
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Problem 18, page 194
Problem 20, page 194
Problem 21, page 195
Problem 22, page 195
Large quantities of ammonia are liberated from urea at hog feed lots in the Midwest. It is not unusual for
local lakes and ponds downwind of these feed lots to absorb considerable amounts of NH 3(g) from the air.
Consider a small lake downwind from such a feed lot where the lake water has a total soluble ammonia
(NH4+ plus NH3(aq)) concentration of 5 x 10-4 M and a pH of 8.55.
(a)
(b)
(c)
6.
Estimate the partial pressure of NH3(g) required to establish this concentration.
pKa = 9.24 for the ionization of NH4+ and Henry’s Law constant (KH) for NH3(g) is 28 mole/l.atm
at 25o C. You may assume activity corrections are negligible.
Construct a pC-pH diagram for the above open system.
With the partial pressure determined in (a), it is not possible for pure water in equilibrium with
NH3(g) to have the given pH. Therefore, some acid or base must have been added to the lake to
obtain that pH. Which, acid or base, has been added and how much (in eq/l)?
An acid mine drainage ([H2SO4}T = 10-4 M, PC02 = 10-1.5 atm) mixes 1:2 with a stream characterized by
PCO2 = 10-1.5 atm, pH = 6.0.
What are the alkalinity and the pH of
(a) the acid mine drainage,
(b) the stream
(c) the mix at the point of confluence
(d) the mix after cascading down the hill and equilibrating with the atmosphere, and
(e) the mix after standing in a small lake and supporting growth of 3 mg/L of algal biomass (NO 3- is used as
the N source and the biomass is given by CH2O and the denitrification reaction is given by :
4NO3- + 5 CH2O + 4H+ == > 2N2 + 5 CO2 + 7H2O)
For CE 520 Students
7.
The acid sulfite pulping process uses sulfurous acid, H2SO3 to attack lignin that holds the wood fibers
together. The fibers are then released and processes into paper products and a hot solution containing
acetic acid, sulfurous acid and some larger organic molecules is generated as a waste. This waste solution
is partially evaporated and then condensed. The condensate can be treated in an anaerobic biological
treatment process, but only of the pH is maintained near neutrality (pH 7). When the process is operating
successfully, the microorganisms mediate the following reactions:
CH3COOH < = > CH4 + CO2
4 H2SO3 + 3 CH3COOH < = > 4H2S + 6H2CO3
(a) Find the pH of a solution of 5,000 mg/L of HAc and 300 mg/L H 2SO3 as S.
(b) What would the solution be if all the HAc and all the H2SO3 were destroyed according to the above
reactions?
(c) How much NaOH, in moles per liter would be needed to bring the initial solution pH into the
range of good treatability (pH 7).
How much Na2CO3 would be required to accomplish the same result.
Comment on the relative requirements of these two bases, considering their relative strengths and the
number of protons each can accept.
(d) If the solution pH were increased to 7.0 with Na 2CO3 and the biological reaction then proceeded to
destroy one-half of the sulfite and all the acetate according to the reactions shown, what would be the final
solution pH be?