UH Dissolution in Chemistry Worksheet

1- What is the pH of a solution saturated with CaCO3 closed to the atmosphere?

2-What is the pH of a solution saturated with CaCO3 open to the atmosphere?

3- For a solution saturated with calcite (CaCO3), how does the solubility of Ca+2 change as a function of pH from 0 to 14 in open and closed systems?

4- Show how the solubility of Fe+2 changes as a function of pH from 0 to 14 given the following recipe [Fe-II] = 10-4 M and a total carbonate conc. (CT) = 10-3 M? Assume closed system.

5-Calculate the composition of a groundwater in equilibrium with PCO2 = 10-1.5 atm, calcite and kaolinite. Disregard all other potential solids.

6-Assuming an open system at equilibrium, only the carbonate system being important, an alkalinity of 1E-3 M and a pH of 9, would calcite precipitate (would solid calcite be present) if the calcium concentration were 1E-3 M?

7-A water contains 135 mg/L Ca+2 and 50 mg/L Mg+2. After the water is softened, the pH = 11 and the carbonate concentration [CO3-2] = 2.6E-5 M. Assume calcium precipitates as CaCO3 (pKs =8.40) Assume magnesium precipitates as Mg(OH)2 (pKs =10.6)

a) Calculate the total hardness (mg CaCO3 mg/L) in the water before softening.b) Calculate the theoretical total hardness remaining in the water after softening based on the solubility of CaCO3 (s) and Mg(OH)2 (s).8- A metal plating factor produces a waste containing soluble zinc with a flow of 100 L/min for 8 hours per day. The waste is collected in a large tank. At the end of each day, the pH of the waste is raised to precipitate out the zinc. The sludge is removed and the pH of the supernatants is lower to pH 6 so that is can be discharged to the local sewer system.a)Consider the removal of zinc from this waste by precipitation as zinc hydroxide (Zn(OH)2, pKs = 16.7). The initial concentration of zinc is 0.01 M. Calculate the pH that the wastewater should be raised in order to reduce the soluble zinc concentrations tob)One day the operator forgets to remove the sludge before the pH is lowered to 6. He notices that all the sludge disappeared when the pH was lowered to 6. He proudly tells his supervisor that he discovered a method to eliminate the need to dispose of their metal sludge in an expensive landfill. Qualitatively, explain the operator’s amazing discovery. 0.05 mg/L (the EPA limit).

Complexation HW
1a.
Calculate the inorganic speciation (percent distribution of dissolved species) of
nickel and mercury (1E-10 M) in an oxic freshwater using major ion composition given
below at pH 8.1. Ignore all potential solids.
Species
Freshwater (M) Seawater (M)
2.8×10-4
TOTNa
TCa
3.7×10-4
1.6×10-4
TOTMg
TOTK
6.0×10-5
2.0×10-4
TOTCl
TOTSO4
1.0×10-4
1.0×10-3
TOTCO3
4.68×10-1
1.02×10-2
5.32×10-2
1.02×10-2
5.45×10-1
2.83×10-2
2.38×10-3
1b.
Assuming the total carbonate concentration (CT) is constant, show the effect
of pH on nickel and mercury speciation at pH 5,7 and 9. Ignore all potential
solids.
1c. At what pH, if any, would Ni or Hg solids precipitate if [Ni]T = [Hg]T = 1E-7 M?
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2. Calculate the speciation and “free” concentrations of EDTA, Fe(III) and Cd(II)
in a
[EDTA]
=
5E-5
model seawater (see seawater composition above) solution containing
T
M, Fe(III) T = 1E-8 M and CdT = 1E-10 M. Ignore all potential solids.
3.Calculate the Cu(II) speciation at pH 5, 7, and 9 in a freshwater lake having the
following major ion composition: NaT = CaT = MgT = KT = ClT = [SO4] T = [CO3] T = 1E-3
M and containing 10-6 M organic complexing agent Y (see reactions below). Assume
[Cu(II)]T = 1E-7 M. Ignore all potential solids.
HY = H+ + YCuY+ = Cu+2 + YCaY+ = Ca+2 + YMgY+ = Mg+2 + Y-
pKa = 6.0
pK = 7.0
pK = 2.0
pK = 2.0
4.Show a p[Cu(II)]T vs pH diagram for a solution having the same composition as
problem 3 but allow solids to precipitate. Also show on the graph the dissolved
copper species making the biggest contribution to the total dissolved copper
concentration (Cu(II)]T).
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Turn in all appropriate MINEQL input, assumptions and output. Make sure
that each question is answered and that the final answer(s) is highlighted
on the MINEQL output.
Section 1: Acid/base/alkalinity
1. Use MINEQL to:
a) Generate a log C vs pH diagram for a 10-3 M phosphate (PO4) system
b) Determine the pH of the following solution at
equilibrium: Recipe: [Na2HPO4] = 10-3 M, [HCl] = 10-3
M
Note: Turn in a pC-pH diagram and output showing the pH of solution at
equilibrium
2. Find the pH of the following solution at equilibrium.
Recipe: [NH4Cl]T = 10-3 [NaOH] = 10-3, PNH3 = 10-6
atm Species: H2O, H+, OH-, Na+, Cl-, NH4+, NH3(aq),
NH3(g)
Reactions: NH4+ = NH3(aq) + H+ pKa = 9.2
NH3(g) = NH3(aq) pKH = -1.8
3. Calculate the alkalinity of a carbonate solution in equilibrium with the
atmosphere (PCO2 = 10-3.5) and a having a pH = 7.3. Assume only
carbonate system is important.
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Section 2: Precipitation and dissolution problem
a) Generate a solubility diagram (p[Fe] dissolved vs. pH) from pH 0 to 14
for a system in equilibrium with solid goethite (FeOOH (s)) using
MINEQL. Show only dissolved Fe+3 species and Fe+3 total dissolved.
b) In the closed, model freshwater system described below, having CaT =
10-4 M and CT = 10-2 M, at what pH, if any, will calcite (CaCO3)
precipitate? Ignore all other solids. Show appropriate MINEQL output to
support your answer.
TOTNa = 2.8E-4 M
TOTMg = 1.6E-4
TOTCl = 2.0E-4
TOTCO3= 1E-2
pH = ?
TOTCa = 1E-4
TOTK = 6.0E-5
TOTSO4 = 1.0E-4
c) At what [Ca+2] concentration, if any, would calcite precipitate in a
closed freshwater with the following composition? Ignore all other
solids. Show appropriate MINEQL output to support your answer.
Freshwater model:
pH = 8.1
TOTCa = ?
TOTK = 6.0E-5
TOTSO4 = 1.0E-4
TOTNa = 2.8E-4 M
TOTMg = 1.6E-4
TOTCl = 2.0E-4
TOTCO3= 1E-3
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Section 3: Complexation chemistry problem
4. Using the models specified below, determine the total solubility of Cd(OH)2(s)
expressed as [Cd+2 T (aq)] in seawater and freshwater (total refers to the sum of
all dissolved Cd+2 forms). Ignore all other solids. Which type of water yields the
highest total dissolved concentration of cadmium [Cd+2 T (aq)]? Show appropriate
MINEQL output to support your answer.
Seawater model:
TOTNa = 4.68E-1 M
TOTMg = 5.32E-2
TOTCl = 5.45E-1
TOTCO3= 2.38E-3
pH = 8.1
TOTCa = 1.02E-2
TOTK = 1.02E-2
TOTSO4 = 2.82E-2
Freshwater model:
TOTNa = 2.8E-4 M
TOTMg = 1.6E-4
TOTCl = 2.0E-4
TOTCO3= 1E-3
pH = 8.1
TOTCa = 1E-4
TOTK = 6.0E-5
TOTSO4 = 1.0E-4
5. Using the seawater model specified in the previous problem, determine the
inorganic speciation of mercury (II) (i.e. calculate the % distribution of all
dissolved Hg+2 species). Assume no solids and a Hg+2 concentration of 1E-10 M.
6. A freshwater lake with the simple composition
NaT = CaT = MgT = KT = ClT = [SO4] T = [CO3] T = 10-3 M
Contains an organic complexing agent X- characterized by
HX = H+ + XCuX+ = Cu 2+ + XCaX+ = Ca2+ + XMgX+ = Mg2+ + X-
log Ka = -7.0
log K = -10.0
log K = -2.0
log K = -2.0
Calculate the dissolved copper speciation at pH 9 when CuT = 10-8 M, XT = 10-6
M. Do not consider solid phases or ionic strength effects.
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Section 4. Redox
7.
Assume the equilibrium pe in an aquatic system is controlled by the
concentrations of SO4-2 and HS–. Calculate the pe of the system, [Fe+2 T] and
[Fe+3 ] if [SO ] = 10 M and [HS] = 10 M, pH = 8, and [FeT] is 2E-6. Ignore all
solids.
8. Determine which form of total dissolved iron (Fe+2 T or Fe+3 T) would dominate
if the pH = 8 and pe = 4.0 and FeT = 2E-6. Ignore all solids.
9. Assume a normal earth atmosphere (pO2 = 10-0.7 atm), pH =8, and that the
copper species are in equilibrium with the O2/H2O couple. What are the
predicted equilibrium concentrations of Cu+1 and Cu+2 under these conditions for
a solution containing 10-4 M total copper? Ignore all solids.
10. Assume a normal earth atmosphere (pO2 = 10-0.7 atm), pH =8, and that the
aqueous sulfur species (SO4-2 and HS-) are in equilibrium with the O2/H2O
couple. Calculate the equilibrium concentrations of the three sulfur species under
these conditions for a solution containing 10-5 M total sulfur?
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