Environmental science Alkalinity homework
Chemistry 431: Environmental Chemistry
Spring 2016
Important Aqueous Chemistry Homework
The goal of this assignment is to review some essential skills covered in Chapter 3 and Chapter 4. Show
all work for your calculations. Submit for a grade by February 24, 2016 by 5:00 PM.
1.) Water with a total alkalinity of 2.00 x 103 eq/L has a pH of 7.0. Calculate the [CO2 (aq)],
[HCO3

], [CO3
2] and [OH
].
2.) For a solution having 1.00 x 103 eg/L total alkalinity (contributions from HCO3

, CO3
2 and
OH
) at pH 10.32, what is the percentage contribution to the alkalinity from CO3
2?
3.) Nitriloacetic acid (NTA) may exist in solution as any of four species, H3T, H2T
, HT2 and T
3
, depending upon the pH of the solution. Using the problem solving strategy shown for the
CO2/HCO3

/CO3
2 system in your textbook and Excel, illustrate graphically the distribution of the
NTA species as a function of pH. That is, plot the fraction of each NTA species (αH3T, αH2T,
etc.) as a function of pH. Show the derivation of each α equation that you use in your Excel file.
4.) Calculate the [PbT
]/[HT2] for NTA in equilibrium with PbCO3 in a medium having [HCO3

]
= 3.00 x 103M.
5.) If the medium in Problem 4 contained excess calcium such that the concentration of uncomplexed
calcium, [Ca2+], were 5.00 x 103 M, what would be the ratio [PbT
]/[CaT
]?
6.) What is the fraction of NTA present as HT2

after HT2 has been brought to equilibrium with solid
PbCO3 at pH 7.0 in a medium in which [HCO3

] = 1.25 x 103M.
7.) Assuming a bicarbonate ion concentration [HCO3

] 0f 1.00 x 103 M and a value of 3.5 x 1011 for the
solubility product constant of FeCO3, what would you expect to be the stable iron species at pH 9.5 and
pE 8.0. Use the pHpE diagram shown in Figure 4.4 in your textbook.
8.) Assuming the partial pressure of oxygen gas is that of atmospheric oxygen 0.021 atm, rather than the
1.00 atm assumed in the derivation of Equation 4.49 in your textbook, derive an equation describing the
oxidizing pE limit of water as a function of pH.
9.) Using the equation derived in Problem 8 and Excel, plot logPO2 as a function of pE at pH 7.00.
10.) Calculate the values of [Fe3+], pE and pH at the point in Figure 4.4 where Fe2+ is at a concentration of
1.00 x 105M, and Fe(OH)2 and Fe(OH)3 are in equilibrium.
11.) Using the Nernst equation, what is the pE value of an acid mine water sample having [Fe3+] = 7.03 x
103M and [Fe2+] = 3.71 x 104M?
12.) How would you expect pE to vary with depth in a stratified lake?