Strong and weak behavior

The equilibrium constant of the protolysis reaction

is known as the acidity constant or the acid dissociation constant (K_a) of HA:

It is frequently expressed as a pK_a value, described as

(Note: a larger K_a value that corresponds to a smaller pK_a.) A selection of K_a and pK_a values is provided in Table 1. If pK_a<0  (that is  K_a>1) the equilibrium lies strongly to the right and HA is termed as strong acid. Acids with pK_a>0 (i.e. K_a<1) are

Table 1. Some acidity constants in water at 25°C

weak acids and go through only partial protolysis. Strong acids in water involve HCl and H₂SO₄, where HF and HSO₄^- are weak acids.

In a identical way it is possible to describe the basicity constant K_b and the subsequent pK_b from the equilibrium We can differentiate strong bases with pK_b<0 and the equilibrium lying to the right side (instances being O^2- and NH^-₂ ) and weak bases with pK_b>0 (example NH₃ and F^-, with pK_b equivalent to 4.75 and 10.55, correspondingly). Though, the make use of pK_b is not necessary, like the base reaction above might be merged with autoprotolysis to depict that in which pK_a refers to the conjugate acid BH⁺. So the pK_a values in Table 1 can be employed to measure the pK_b values for the conjugate bases A^-.

As a strong acid like HCl is fully protolyzed it is not possible to study this species itself in water. H₃O⁺ is efficiently the strongest acid feasible there and any stronger acid is said to be leveled. In the same way, strong bases like NH^-₂ are leveled to the strongest base feasible in water, OH^-. Solvent leveling limits the range of acid-base nature which can be observed in a given solvent and is one reason for using another solvents with distinct leveling ranges. For instance, liquid ammonia is very basic that is compared with water and H₂SO₄ is very acidic.