Homework 3 –Speciation, Acid Dissociation and Carbonate Chemistry
Aqueous Geochemistry
1. Mg2+ can form complexes with HCO3- and SO42-. The reactions for the formation of these
complexes are:
)(
33
2aq
MgHCOHCOMg
and
)(
0
4
2
4
2aq
MgSOSOMg
The logKassoc. for the reactions are 1.07 and 2.37, respectively. Calculate the estimated
activity of the two complexes by using law of mass action equations for the speciation
reactions. The activities calculated from laboratory measurements for total amount of Mg2+,
HCO3-, and SO42- are given in the table below.
Species Activity (moles/L)
Mg2+ 1.1
10-3
HCO3- 6.1
10-3
SO42- 1.3
10-5
2. The activity of Mg2+ in the table above represents the sum of all of the Mg species in solution
(i.e. the activity of Mg2+ as the uncomplexed ion plus the activity of all Mg2+ complexes).
Mathematically, this can be represented by the formula:
0
43
2MgSOMgHCOMg
species aaaMg
where ΣMgspecies is the measured activity and a’s are the activity of the individual species.
Using the calculated activity of the complexes from Question 1, calculate the estimated
activity of the uncomplexed Mg2+ ion using the formula above.
3. Calculate the percentage that each of the Mg species constitutes of the ΣMgspecies in solution.
4. At what pH is [H3SiO4-] 1 order of magnitude less than [H4SiO4]? In other words, at what pH
does [H3SiO4-] = 0.1[H4SiO4]? Ka = 10-9.83
5. What are the dominant phosphorous species (i.e. H3PO4, H2PO4-, HPO42-, or PO43-) at pH’s of
6, 7, and 8? Ka1 = 10-2.15, Ka2 = 10-7.20, Ka3 = 10-12.35
6. A groundwater sample has a pH of 7 and an alkalinity of 340 mg/L as CaCO3. Calculate its
HCO3- activity.
7. Using the HCO3- activity calculated from the previous question, calculate the PCO2 (in
atmospheres) of the sample. Ka1 = 10-6.35 and KCO2 = 10-1.47 moles/(L*atm)
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