Advanced Geochemistry of Natural Waters
GEOL 478/578
CALCULATIONS USING THE MINEQL+ COMPUTER PROGRAM
All calculations to be performed here will be conducted using the computer program
MINEQL+. This program can perform a number of different types of speciation and solubility
calculations.
Problem 1:
Sufficient strong acid is added to completely dissolve 10-3 moles of CaCO3(s) into a liter of
water in equilibrium with atmospheric CO2. The final pH is measured to be 4.5.
a) What is the concentration of all aqueous species?
b) What is the alkalinity?
c) How much strong acid was added?
d) Is the final solution saturated with respect to any solids (calculate saturation indices)?
Do parts a)-d) first assuming activity coefficients are equal to zero, and then repeat using activity
coefficient corrections.
Problem 2:
Repeat problem 1 assuming that the solution also contains 0.05 M NaCl in addition to the
components described above.
Problem 3:
You have analyzed a natural water and found it to contain 4.1x10-5 M Ca, 3.1x10-5 M Al and
1.3x10-4 M SO42-. Assuming charge balance, calculate the theoretical pH this system should have, and
compare it to the measured pH = 3.69±0.05. If the calculated and measured pH’s do not agree, how
do you explain this?
Problem 4:
a) Calculate the pH of a 0.1 M solution of NaHCO3 neglecting ion pairs and activity coefficients.
b) Calculate the pH of a 0.1 M solution of NaHCO3 including ion pairs and activity coefficients.
Problem 5:
Calculate the solubility of calcite in a 0.07 M solution of NaCl in equilibrium with atmospheric
CO2 at pH = 8.0. How much excess acid is required to obtain these conditions? Include activity
coefficients in your calculations.
