The excess HCl is titrated with NaOH(aq) until enough OH{ (from the NaOH solution) has
been added to completely react with the excess H+ (from the excess HCl in the solution). So, part
of the added acid is neutralized by the antacid tablet; the remainder is neutralized by the NaOH
added. This is called back titration.
The equivalence point is when the number of moles of NaOH added equals the number of moles
of HCl remaining after the reaction with the tablet. HCl is the H+(aq) source; NaOH is the OH{(aq)
source. At the endpoint of the titration, the acid has been neutralized by the base.