Complex sentence: Pempidine is an aliphatic, sterically-hindered, cyclic, tertiary amine, which is a weak base: in its protonated form it has a pK a of 11.25.
Split into simpler sentences: Pempidine is an aliphatic, sterically-hindered, cyclic, tertiary amine. It (in its protonated form) has a pK a of 11.25.