Stoichiometry: pH of Strong Acids and Strong Bases

The strength of an acid or a base is determined by the value of equilibrium acid constant (Ka). The greater Ka, the stronger acidity. Conversely, a strength of a base determined by the the value of equilibrium base constant (Kb). The greater Kb, the greater properties of the base.

Strength acid and strength base closely related to the ability of the acid or base ionized in the water. Strong acid and strong base fully ionized in the water, while the weak acid and weak base partial ionized in the water. The amount of ionized mole on the weak acid or base is vary depending on the size of the degree of ionization rates (α).

Acids that includes a strong acid is HCl, HBr, HI, H₂SO₄, HNO₃, HClO₄. In addition, other acids can be considered as a weak acid. That acids is mono valence except H2SO4 bivalen according to the number of its H⁺. Its H⁺ concentration can be calculated using the formula:

[H⁺] = M x val
where:
[H⁺] = concentration of H⁺
M = concentration of acid
val = acid valency
Its acid pH can be calculated using the formula:
pH = - log [H₊]

Example: Calculate pH 0.2 M HCl
[H⁺] = M.val = 0.2 M x 0.2 M = 1 = 2x10^-1 M
pH = - log [H⁺] = - log 2x10^-1 = 1 - log 2

While for base, bases which is considered as strong bases is NaOH, KOH, Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂. Other bases can be considered as weak bases. NaOH and KOH is mono valence base. Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂ is bivalen bases according to the number of its OH^-. Concentration of OH can be calculated using the formula:

[OH^-] = M x val
where:
[OH^-] = concentration of OH^-
M = concentration of base
val = base valency
base its pH can be calculated using the formula:
pOH = - log [OH^-]
pH = 14 - pOH

Example: Calculate pH Ba(OH)₂ 0.05 M
[OH^-] = M M.val = 0.05 x 2 = 0.1 M = 1x10^-1 M
pH = - log [OH^-] = - log 1x10^-1 = 1