Solution

a) HCl is a strong acid. When it donates a proton, a Cl^– ion is produced, and so Cl^– is the conjugate base.

b) CH₃NH₂ is an amine and therefore a weak base. Adding a proton gives CH₃NH₃⁺, its conjugate acid.

c) Adding a proton to the strong base OH^– gives H₂O its conjugate acid.

d) Hydrogen carbonate ion, HCO₃^–, is derived from a diprotic acid and is amphiprotic. Its conjugate acid is H₂CO₃, and its conjugate base is CO₃^2–.