Which of the following is considered a weak base?

The identification of a weak base centers on the base's ability to dissociate in water and accept protons (H+). In this case, ammonia (NH3) stands out as a weak base because it does not fully ionize in solution. Instead, it establishes an equilibrium in which only a portion of the ammonia molecules accept protons to form ammonium ions (NH4+) and hydroxide ions (OH-).

This characteristic is what differentiates it from the other options listed. Calcium hydroxide (Ca(OH)2), sodium hydroxide (NaOH), and potassium hydroxide (KOH) are all strong bases—they completely dissociate in water, resulting in a high concentration of hydroxide ions. This complete dissociation reflects the ability of strong bases to significantly raise pH levels in solution, an ability that is not present in the same capacity in ammonia. Therefore, ammonia is categorized as a weak base due to its partial ionization and lower pH influence compared to the strong bases listed.