Weak acids react with water in a process known as acid ionization. The ionization of a weak monoprotic acid (denoted as HA) in water occurs by the following equilibrium,
The acid-ionization constant (Ka) is the equilibrium constant for the ionization of a weak acid. The value of Ka tells you how much a weak acid dissociates in water, thus indicating how much H+ is formed. The acid ionization constant is defined as,
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In the case of a diprotic acid, an acid that loses two protons in aqueous solution, working with the ionization constant involves the use of quadratic functions. Take oxalic acid, H2C2O4 is a diprotic organic acid found in many plants, notably rhubarb leaves. In humans, oxalic acid can be poisonous, causing skin and mouth irritation, nausea, vomiting, diarrhea, kidney stones, and, rarely, death. |
Oxalic acid has the following simultaneous acid-ionization equilibria
with the following acid-ionization constants at 25 °C,
Notice that oxalic acid has two ionization constants, Ka1 and Ka2, because it is a diprotic acid.
Use the information provided to answer the following questions:
Determine the equilibrium concentration of H+.
Determine the equilibrium concentration of C2O42-.
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