Answer
$HC{O_3}^-$ is acting as an acid and $HS^-$ is acting as a base:
$$HC{O_3}^-(aq) + HS^-(aq) \leftrightharpoons C{O_3}^{2-}(aq) + H_2S(aq)$$
$HS^-$ is acting as an acid and $HC{O_3}^-$ is acting as a base:
$$HC{O_3}^-(aq) + HS^-(aq) \leftrightharpoons H_2C{O_3}(aq) + S^{2-}(aq)$$
Work Step by Step
1. Write the first equation. Since $HC{O_3}^-$ is acting as an acid, it must donate one proton to the other molecule, and since $HS^-$ is acting as a base, it must accept one proton from the other substance.
2. For the second one, we just invert the roles. $HC{O_3}^-$ will receive one proton and $HS^-$ will donate one.
