To know if a base is strong or weak by formula, the primary consideration is the degree of ionization or dissociation of the base in solution:
- A strong base is one that is 100% ionized in aqueous solution. This means it completely dissociates into hydroxide ions (OH⁻) and its corresponding cations. Strong bases are typically hydroxides of alkali metals and heavier alkaline earth metals, such as NaOH, KOH, Ca(OH)₂, and Ba(OH)₂.
- A weak base is less than 100% ionized in solution. It does not fully dissociate to give hydroxide ions. Examples include bases like NH₃ (ammonia), which do not contain OH⁻ ions in their formula but generate hydroxide ions by reacting with water reversibly.
The general formula indication for a strong base is that it contains OH⁻ and belongs to certain groups in the periodic table (alkali metals and heavier alkaline earth metals). For weak bases, the formula may not have OH⁻, and these bases ionize only partially. In summary, with a formula:
- If it is a metal hydroxide from group 1 or heavier alkaline earth metals (like NaOH, KOH, Ca(OH)₂), it is a strong base.
- If it lacks OH⁻ or is something like NH₃, it is a weak base.
This classification is based on the degree of ionization shown in the dissociation formula. Strong bases dissociate 100% in water, weak bases do not.
