1, strong acid can completely ionize hydrogen ions in water, so it is impossible to distinguish the strength of acidity.

However, in glacial acetic acid, they cannot be completely ionized, and acidity can be distinguished according to ionization degree.

take for example

H2SO4+HAc = (reversible) = H2AC++HSO4-Ka = 6.3× 10-9.

HClO4+HAc= (reversible) = H2AC++Clo4-Ka =1.6×10-6.

HClO4 has a large Ka, so it has a high degree of ionization and is more acidic than H2SO4.

2. The simplest form requires c/Ka > 500. When the degree of ionization of acid is very small, it can be considered that the concentration of HA at equilibrium is the initial concentration.

When c/ka

This is explained in the derivation of the formula. Read this book by yourself.