The pKa is the strength of a weak acid. The Ka is the dissociation constant of an acid. Now, for a weak acid, the dissociation constant is too small. This makes it difficult to use in equations and calculations. Hence, the pKa is used as representation of the Ka of weak acids, as it is a more manageable number.
To convert pKa into Ka, all you need to do is find the log of pKa. This is because the equation relating pKa with the dissociation constant is pKa=-logKa. When you want to find the pKa, you can simply find the antilog of the Ka.
