5. Calculate the pH of a 0.240 M solution of sodium bicarbonate.
Write the appropriate reactions that incorporate the bicarbonate species?
Give the students about five minutes to begin working on this problem. They will probably identify sodium bicarbonate as both a weak acid and a weak base.
Write an expression for H3O+ in terms of the other species?
Groups should eventually recognize that for every one mole of CO32- formed, one mole of H3O+ was formed, and for every mole of H2CO3 formed, one mole of H3O+ was used up. They may need some prompting to think about what will occur for the H2CO3. Once the groups have the correct expression, spend time at the board going through the algebraic manipulation that incorporates this expression into the Ka expressions to arrive at the formula that can be used to calculate the pH.
At the end of this problem, it is beneficial to most students to spend several minutes generalizing the methods used in the last five problems. Address how to approach problems of each of the following starting species: a fully protonated acid, appreciable amounts of neighboring intermediates, appreciable amounts of varying intermediates, one intermediate, a fully deprotonated base.