In an acid-base equilibrium $\mathrm{HA}+\mathrm{B}{}^{-}\rightleftharpoons \mathrm{A}{}^{-}+\mathrm{HB}$, the equilibrium lies on the side of

Proton transfer is favoured from the stronger acid to the stronger base, producing the weaker acid and the weaker base (higher $\mathrm{p}{K}_a$ acid, more stable and less basic conjugate base). Hence the equilibrium lies toward the weaker acid and weaker base.