Question

Consider the following reaction:
{\rm{HC}}_2 {\rm{H}}_3 {\rm{O}}_2 \left( {aq} \right)\; +\;{\rm{H}}_2 {\rm{O}}\left( l \right)\; \rightleftharpoons\;{\rm{H}}_3 {\rm{O}}^ + \left( {aq} \right)\; + \;{\rm{C}}_2{\rm{H}}_3 {\rm{O}}_2 ^ - \left( {aq} \right)
K_{\rm{c}} = 1.8\; \times \;10^{ - 5} at 25^\circ {\rm C}

If a solution initially contains 0.220 M {\rm{HC}}_2 {\rm{H}}_3{\rm{O}}_2, what is the equilibrium concentration of {\rm{H}}_3{\rm{O}}^ + at 25^\circ {\rm C}?