Acid strength is an acid's ability or tendency to lose a proton. Different acids should have different strengths. There are very few strong acids which are completely ionized in water or solution by loosing one proton. Strong acids are strong electrolytes. In other words, one mole of strong acid HA dissolves completely in water, yielding one mole of H+ ion and one mole of conjugate base A-. A strong acid does not have any non ionized acid of HA.
According to the equation
HA(aq) →→ H+(aq) + A-(aq)
There are several useful definitions of acids and bases. The simplest is the Arrhenius definition, which says that an acid is any chemical that produces hydronium ions in water, while a base is any chemical that produces hydroxide ions in water. The hydronium ion, H3O+ (aq), is sometimes written H+(aq), or just H+, for the sake of brevity.
The two equations on your screen involving HCl are just two ways of describing the same reaction. The H+ ion – also called a proton – is always attached to some other molecule or ion. In aqueous solutions, it attaches itself to a water molecule to form the hydronium ion, H3O+. We will usually just write H+.
Strong acids in water
In aqueous solution, a strong acid is an acid that ionizes completely by losing one proton. The proton lost is captured by a water molecule to form a hydronium ion:
HA(aq) + H2O → H3O+(aq) + A−(aq)
For sulfuric acid, which is diprotic, the "strong acid" designation refers only to dissociation of the first proton
H2SO4(aq) → H+(aq) + HSO4−(aq)
The acid must be stronger in aqueous solution than hydronium ions, so strong acids are acids with a pKa < −1.74. An example is HCl for which pKa = −6.3. This generally means that, in aqueous solution at standard temperature and pressure, the concentration of hydronium ions is equal to the concentration of strong acid introduced to the solution.
Common strong acids
This is a list of strong acids with pKa < -1.74, which is stronger than the hydronium ion, from strongest to weakest.
• Perchloric acid HClO4 (pKa = −10)
• Hydroiodic acid HI (pKa = −9.3)
• Hydrobromic acid HBr (pKa = −8.7)
• Hydrochloric acid HCl (pKa = −6.3)
Examples for strong acids which dissolves or ionizes 100% in water
HCl - Hydrochloric acid
HI - Hydro iodic acid
HNO3 - Nitric acid
H2SO4 - Sulfuric acid