2. Calculate the pH of a 0.089 M solution of sodium carbonate.
Before starting this problem, talk about nomenclature for polyprotic acids. For example:
H3PO4 phosphoric acid
NaH2PO sodium dihydrogen phosphate
Na2HPO4 disodium hydrogen phosphate
Na3PO4 sodium phosphate
What are the two equilibria are occurring in this solution? What are the chemical expressions representing them?
Students may be tempted to initially say that the presence of sodium will result in the formation of sodium hydroxide which could shift the equilibrium. Remind them when ions can be assumed to be spectators. It should take the students about five minutes to write the correct expressions and recognize that they correspond to the Kb expressions. Make sure that the students determine the Kb1 and Kb2 values from the correct Ka values.
Can the second expression be ignored?
Students should recognize that a large difference in Kb values implies that the second reaction is negligible. Point out that this is just the opposite of the previous scenario; in this case we started with all of a deprotonated polyprotic acid.
What is the pH of the solution?