# Solutions of Polyprotic Acid/Base Systems, Problem B

**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:

H_{3}PO_{4} phosphoric acid

NaH_{2}PO sodium dihydrogen phosphate

Na_{2}HPO_{4 }disodium hydrogen phosphate

Na_{3}PO_{4 }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 *K _{b}* expressions. Make sure that the students determine the

*K*and

_{b1}*K*values from the correct

_{b2}*K*values.

_{a}*Can the second expression be ignored?*

Students should recognize that a large difference in *K _{b}* 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?*