Question

The table below provides percent ionization values to indicate the extent that 0.1M solutions of certain weak acids ionize in water according to the generic equation, HA + H2O Imported Asset H3O+ + A−. How are percent ionization values similar to equilibrium constant values? Explain your reasoning and identify which weak acid would have the smallest equilibrium constant.

Weak Acid:
1. HNO2
2. HClO2
3. HCN

Percent Ionization:
1. 6.2%
2. 28.2%
3. 0.007%

Answer 1

The acid dissociates as following:
HA + H₂O ↔ H₃O⁺ + A⁻
The value of percent ionization shows how much of HA molecules has dissociated, while the equilibrium constant value is calculated as follows:
K = [H₃O⁺] x [A⁻] / [HA]
The concentrations of H₃O⁺ and A⁻ depend on the amount of HA that has dissociated, same as the value of percent ionization, so the values of percent ionization and equilibrium constant are similar because they describe the same process. Comparing the values of ionization percent can be also used to determine if some acid is weak or strong, and to compare the strength of different acids, the same as equilibrium constant.

The smallest the values of ionization percent the acid is weaker.
Because 0.007% < 6.2 % < 28.2%, the strength of the acids increase as:
HCN < HNO₂ < HClO₂, which means that HCN is the weakest, while HClO₂ is the strongest of the listed acids.