A graduate student is preparing a solution in the laboratory. She mixes 10-4 mole NaHCO3 (s) and 10-4 mole Na2CO3 (s) into water for a total solution volume of 1 L. Assume that the solid NaHCO3 and Na2CO3 dissolve completely, and that the solution is not open to the atmosphere. In this system, the following equilibrium reactions are relevant: H2CO3 <--> H++ HCO3-1 pKa = 6.3
HCO3-1 <--> H+ + CO32- pKa = 10.3
H2O <--> H+ + OH- pKw = 14
a. What is the concentration of Na+ added to this system?
b. What is the total concentration of carbonate species added to the water? (Total concentration of carbonate species added = [H2CO3*]added+[HCO3 - ] added +[CO3 2- ] added)
c. The added concentrations of the carbonate species (H2CO3*, HCO3 - , and CO3 2- ) used in part b are not the final equilibrium concentrations of these species. Write a mass balance equation on the three carbonate-containing species at equilibrium.
d. Write the 3 equilibrium expressions relevant to this system (i. e., K=activity of the products/activity of the reactants).
e. Make a list of the positive ions in this system.
f. Make a list of the negative ions in this system.
g. Using your answers from parts e and f, write the charge balance for the system.
h. You should now have 5 equations (from parts c, d, and g) and 5 unknowns. Solve for the equilibrium pH.