Using PHREEQC, I want to model a reaction in which the reactants are CaCO3 and H2SO4. While the amount of CaCO3 is to be kept constant (say 12g/kg of water), the amount of H2SO4 is to be changed (say 0.1, 0.07, 0.03, 0.01 M). Can somebody suggest how to write this in the program?