# PHREEQC: Molality and mole calculations

If you hate doing mole calculations by hand, check out this PHREEQC tip. Good thing about the free software by USGS is that you can input the variables as mg/L (or other units) and it will first speciate the solutions to give you the molality and moles for each component.

Lets just use one example I have been working for last couple of days. I was trying to calculate amount of Ca+ required  in the groundwater to result the amount of Na+2 measured in the sample. If we assume 100% ion exchange, then the calculation becomes simple. All you need is to figure out the mole ratio of Na+ and Ca+.

For this particular example, I actually did not use PHREEQC at all other than just getting my mole numbers. Well, I noticed that we have large charge balance error. Means I don’t have every component of the solution. Search my guide for Charge Balance Error to understand how PHREEQC could be used to determine the quality of a sample.

Enjoy. If you don’t want PHREEQC, use my free spreadsheet to calculate mole ratios.

==========================Input Code===================

TITLE Example
SOLUTION 1
units   ppm
Ca              1
Mg              .26
Na              506
K               .9
Alkalinity      380.64 as HCO3
END
===========================================================

Note: You can add all of your cations and anions here. Once you are done, hit “RUN”

==========================Out Put===========================

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——————

SOLUTION_MASTER_SPECIES
SOLUTION_SPECIES
PHASES
EXCHANGE_MASTER_SPECIES
EXCHANGE_SPECIES
SURFACE_MASTER_SPECIES
SURFACE_SPECIES
RATES
END
————————————
Reading input data for simulation 1.
————————————

DATABASE C:\Program Files\USGS\Phreeqc Interactive 2.15.0\phreeqc.dat
TITLE Example
SOLUTION 1
units   ppm
Ca              1
Mg              .26
Na              506
K               .9
Alkalinity      380.64 as HCO3
END
—–
TITLE
—–

10 East (Underground) Sampled on 5/13/10

——————————————-
Beginning of initial solution calculations.
——————————————-

Initial solution 1.

—————————–Solution composition——————————

Elements           Molality       Moles

Alkalinity       6.244e-003  6.244e-003
Ca               2.497e-005  2.497e-005
K                2.304e-005  2.304e-005
Mg               1.070e-005  1.070e-005
Na               2.203e-002  2.203e-002

—————————-Description of solution—————————-

pH  =   7.000
pe  =   4.000
Activity of water  =   0.999
Ionic strength  =  1.416e-002
Mass of water (kg)  =  1.000e+000
Total carbon (mol/kg)  =  7.471e-003
Total CO2 (mol/kg)  =  7.471e-003
Temperature (deg C)  =  25.000
Electrical balance (eq)  =  1.588e-002
Percent error, 100*(Cat-|An|)/(Cat+|An|)  =  56.23
Iterations  =   9
Total H  = 1.110187e+002
Total O  = 5.552740e+001

—————————-Distribution of species—————————-

Log       Log         Log
Species                 Molality    Activity  Molality  Activity     Gamma

OH-                   1.131e-007  1.001e-007    -6.946    -7.000    -0.053
H+                    1.109e-007  1.000e-007    -6.955    -7.000    -0.045
H2O                   5.551e+001  9.995e-001     1.744    -0.000     0.000
C(4)            7.471e-003
HCO3-                 6.171e-003  5.500e-003    -2.210    -2.260    -0.050
CO2                   1.233e-003  1.237e-003    -2.909    -2.908     0.001
NaHCO3                6.017e-005  6.036e-005    -4.221    -4.219     0.001
CO3-2                 4.089e-006  2.579e-006    -5.388    -5.589    -0.200
CaHCO3+               1.178e-006  1.049e-006    -5.929    -5.979    -0.050
NaCO3-                1.057e-006  9.374e-007    -5.976    -6.028    -0.052
MgHCO3+               4.711e-007  4.180e-007    -6.327    -6.379    -0.052
CaCO3                 6.459e-008  6.480e-008    -7.190    -7.188     0.001
MgCO3                 1.593e-008  1.599e-008    -7.798    -7.796     0.001
Ca              2.497e-005
Ca+2                  2.373e-005  1.496e-005    -4.625    -4.825    -0.200
CaHCO3+               1.178e-006  1.049e-006    -5.929    -5.979    -0.050
CaCO3                 6.459e-008  6.480e-008    -7.190    -7.188     0.001
CaOH+                 2.796e-011  2.481e-011   -10.553   -10.605    -0.052
H(0)            1.411e-025
H2                    7.056e-026  7.079e-026   -25.151   -25.150     0.001
K               2.304e-005
K+                    2.304e-005  2.039e-005    -4.638    -4.691    -0.053
KOH                   7.042e-013  7.065e-013   -12.152   -12.151     0.001
Mg              1.070e-005
Mg+2                  1.022e-005  6.495e-006    -4.991    -5.187    -0.197
MgHCO3+               4.711e-007  4.180e-007    -6.327    -6.379    -0.052
MgCO3                 1.593e-008  1.599e-008    -7.798    -7.796     0.001
MgOH+                 2.657e-010  2.357e-010    -9.576    -9.628    -0.052
Na              2.203e-002
Na+                   2.197e-002  1.952e-002    -1.658    -1.710    -0.051
NaHCO3                6.017e-005  6.036e-005    -4.221    -4.219     0.001
NaCO3-                1.057e-006  9.374e-007    -5.976    -6.028    -0.052
NaOH                  1.285e-009  1.289e-009    -8.891    -8.890     0.001
O(0)            0.000e+000
O2                    0.000e+000  0.000e+000   -42.082   -42.080     0.001

——————————Saturation indices——————————-

Phase               SI log IAP  log KT

Aragonite        -2.08  -10.41   -8.34  CaCO3
Calcite          -1.93  -10.41   -8.48  CaCO3
CO2(g)           -1.44   -2.91   -1.47  CO2
Dolomite         -4.10  -21.19  -17.09  CaMg(CO3)2
H2(g)           -22.00  -25.15   -3.15  H2
H2O(g)           -1.51   -0.00    1.51  H2O
O2(g)           -39.19  -42.08   -2.89  O2

——————
End of simulation.
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————————————
Reading input data for simulation 2.
————————————

———–
End of run.
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==============================================================

Editor

Ankan Basu is a Certified Professional Geologist (CPG) with 10+ years of experience in the field of geology, hydrogeology and geochemistry.