You strong acids are those acids that ionize a lot. Acidis every substance that, second Arrhenius, when suffering ionization, produces a lot of ions. The cation produced is always hydronium [H+ or H3O+].
To define if a acid is strong, we need one of the following parameters:
degree of ionization (α) greater than or equal to 50%
By convention, an acid is considered strong when its degree of ionization is greater than or equal to 50%. This means that for every 100 molecules of an acid considered to be strong added to water, at least 50 of them must be ionized.
To calculate the degree of ionization, just divide the amount of ionized particles by the number of dissolved particles:
Example - By adding 80 particles of an acid to water, 60 of them are ionized. What is the degree of ionization of this acid?
Let's add the values to the expression:
To find this percentage value, simply multiply the result by 100. Thus, the degree of ionization of this acid is 75%, that is, it is a strong acid.
Subtraction between oxygens and ionizable hydrogens in the case of oxyacids (acids with oxygen)
When the result of the subtraction between the number of oxygens and the number of ionizable hydrogens present in the acid molecule is equal to or greater than 2, the acid will be classified as strong.
Example: H4P2O7
x = number of oxygens - number of ionizable hydrogens
X = 7 - 4
x = 3
Since x is greater than 2, so it is a strong acid.
Hydracids (oxygen-free acids)
The only hydroxides that are considered strong acids are hydrochloric acid (HCl), hydrobromic acid (HBr) and hydriodic acid (Hl).
Influence of a strong acid on pH of a solution
As a strong acid produces a lot of H ions+ in the middle, the concentration of these cations in the solution is high, which consequently leads to a low pH.
By Me. Diogo Lopes Dias
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/quimica/o-que-e-acido-forte.htm