We can measure the acidity of compounds by its Ka value. Then, what is the necessity of finding pKa value? Why there is a term called pKa?
The disociation constant is given in the terms by Kd = No of the oxidation product [ H+] / no. of the reduction ion [OH-] of specific species. This value is gvien very decimal point to assumption of acidity ex 10-6 to-9. This value is simplyfied by taking log you can get the values of acidy between 1-14. i.e K is rate constant and a is association or dissociation of the acidity and p is taking a log function, the log function is negative logerthem.
it is easier to use simpler number compared to 10^-5 to 10^-9 values. Similar way to the pH. it is easier to use pH not [H+].
For very small values, it is more efficient and accurate to use the log function.
That is why pKa is determined using pH.
Ka=Kd*[H20]=[A-][H3O+]/[HA]
pH=pKa + log [A-/HA] - Henderson Hasselbalch equation
pH=pKa at half equilibrium, when [A-]=[HA]
Ka is the antilog of this value.
Therefore you actually measure the pKa not the Ka and you back calculate the Ka.
The use of pKa value is advantageousthan Ka to know the ionization state of a compound in solution.pKa is pH valve at which the acidic specie is 50% ionized. Therefore, one can know the %age of cationic ,neutral or anionic species presence at any given pH from its pKa value.However, the Ka valve does not provide direct guess of this information
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