Mixing Acids and Bases Practice – POLY-PROTIC ACID TITRATION – Chemistry 121 Hanson
When an acid has more than one H that can come off, we call it a polyprotic acid. A polyprotic acid with two
acidic hydrogens, such as H2CO3, is called a diprotic acid; a polyprotic acid that has three acidic protons is called
a triprotic acid. The titration curve for a polyprotic acid has more than one plateau and more than one equivalence
point. Consider the data below, involving 0.1 M NaOH being added to 10 mL of a solution containing H2CO3.
Just take these step-by-step. We start with a solution that is predominantly H2CO3. Addition of base removes a
proton, leading to HCO3−, so there is a Ka for that process. You can see this in the first 10 mL of base added. This
species still has an acidic proton, but because it already has a negative charge, it is now harder to remove the next
proton. As more base is added, though, that proton also comes off, and by 20 mL in this example we have another
(somewhat harder to see) equivalence point. At this point, the H2CO3 has pretty much all been converted to CO32−.
1. Write the two acid dissociation equilibrium equations, one involving H2CO3 and the other HCO3−.
2. From the graph, determine the pKa and, from that, K, for each of your two equations.
3. What is the molarity of this acid? [Which equivalent point should you use?]
4. Buffer calculations can be done exactly the same way as for monoprotic acids. You just have to use the
appropriate equilibrium expression for that particular stage in the process. Using pKa1 = 6.36 and pKa2 =
10.25, predict the pH after 8 mL of NaOH have been added and after 13 mL have been added. Check your
answer with the graph.
ANSWERS:
1.
2.
H2CO3 + H2O
HCO3−. + H3O+
HCO3− + H2O
CO32−. + H3O+
H2CO3 + H2O
HCO3−. + H3O+
pKa 6.5
K = 3E-7
HCO3− + H2O
CO32−. + H3O+
pKa 10.4
K = 4E-11
Your values may be a bit different. Actual values I used for pKa to create the graph were 6.36 and 10.25.
3.
0.1 M
4.
see graph.