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1. Calculate the aqueous concentration of Mg+2 in equilibrium with the solid MgCO3 (pKs = 7.46 at
I = 0, T = 25C) when the ionic strength = 1.0 M and the temperature equals 25C using the Davies
equation incorporated into Table 2.3 (attached).
2. What is the pH of a 10-4 M solution of NaHA assuming that NaHA is the salt of a non-volatile
diprotic acid, H2A (pK1 = 6.0, pK2 = 10). Ignore ionic strength effects. Assume T = 25C.
3. Sketch a pC-pH diagram, and use it to find approximate pH of the following solution at
equilibrium. Show all work for full credit.
Recipe: [NH4Cl]T = 10-3 [NaOH] = 10-3, PNH3 = 10-6 atm
Species: H2O, H+, OH-, Na+, Cl-, NH4+, NH3(aq), NH3(g)
NH4+ = NH3(aq) + H+ pKa = 9.2
NH3(g) = NH3(aq) pKH = -1.8
4. Planet Thethys has a nitrogen and hydrogen sulfide atmosphere. Thethysian aquatic chemists
have defined the alkalinity expression with respect to the H2S equivalence point. Assuming only
sulfide system contributes to alkalinity, what is the pH of a Thethysian lake having an alkalinity of
10-3 M, PH2S = 10-3 atm.
5. The upper layer of a thermally stratified lake (epilimnion) has a pH = 7.0, an alkalinity of
10-3M and a boundary layer thickness (Z) of 0.003 cm. Assuming only the carbonate system
contributes to the alkalinity and ignoring any chemical enhancement factor associated with the
chemical reaction of H2CO3, calculate the flux of CO2(g) across the surface of a lake (T =
25˚C) in equilibrium with a PCO2 of 10-3.5 atm. The molecular diffusion coefficient for CO2 is
5 x 10-5 cm2s-1 and KH for CO2 is 10-1.5 M/atm at 25 ˚C.
6. Calculate the pH of a water open to the atmosphere where CT and alkalinity are
approximately equal at 1.6 x 10-3 eq/L assuming carbonate to be the only weak acid. Show all
work and list all assumptions. Solve algebraically or use pC-pH graph.
7. Calculate the pH of a natural water with an alkalinity of 1 x 10-4 eq L-1 at equilibrium with an
atmosphere having a PCO2 = 10-3.5 atm. Show all work for full credit.
H2O = H+ + OH- pKw = 14
CO2(g) + H2O = H2CO3* pKH = 1.5
H2CO3 = H+ + HCO3- pK1 = 6.3
HCO3- = H+ + CO3-2 pK2 = 10.3
8. Calculate the alkalinity of a solution having the following recipe. Assume only the carbonate
system is important. PCO2 = 10-3.5, pH = 7.3. Show all work for full credit.
What is the pH of a solution having the following recipe? Assume open system, PCO2 = 10-3.5
atm. Show all work for full credit. Assume HCl is a strong acid.
Recipe: [Na2CO3] = 10-3, [HCl] = 102
Potentially useful information
For CO2: PCO2 = 10–3.5 atm, KH = 10-1.5 moles/L atm at 25˚C (CO2(g) + H20 = H2CO3*) at 25˚C
For H2CO3: pKa1 = 6.3 and pKa2 = 10.3 at 25˚C
For H2S, K = 0.1 moles/L atm at 25˚C (H2S (g) = H2S (aq))
For H2S, pKa1 = 7.0 and pKa2 = 14 at 25˚C
For H3PO4, pKa1 = 2.1, pKa2 = 7.3, pKa3 = 12.3 at 25˚C
Assume that all salts, acids and bases not specifically listed completely dissociate.
R = 1.987 cal/mole deg = 8.314 J/mole deg = 0.08206 L atm/mol deg
Van’t hoff equation: ln [KT/K T0] = - ∆H/R[1/T – 1/T0]
Jg = 10-3(D/Z)(Cgsat –Cg)
Where Jg is the flux of the gas per unt area (mol cm-2 s-1), D is the molecular diffusion coefficient
of the gas (cm2 s-1), Cgsat is the concentration (mol L-1) of the gas in equilibrium with the given
partial pressure Pg, Cg is the concentration of the gas in the bulk aqueous phase and Z is the
average depth of the water laminar boundary layer through which the diffusion is taking place