You have 25.0 ml hci of unknown concentration. it takes 12.5 of 0.2 m naoh to neutralize the acid. determine the concentration of hci in the following steps. write the balanced equation for the reaction.

The balanced equation for the reaction between NaOH and HCl is 
NaOH + HCl ---> NaCl + H₂O
stoichiometry of NaOH to HCl is 1:1
during neutralisation, number of H⁺ ions = OH⁻ ions. Together they form water molecules and neutralise the solution as water is neutral.
Number of NaOH moles in 1000 ml - 0.2 mol
therefore in 12.5 ml -0.2/1000 x 12.5
                                = 0.0025 mol
at neutralisation point , number of OH⁻ mol = number of H⁺ ions
    since stoichiometry of base to acid is 1:1
number of HCl moles reacted = 0.0025 mol
concentration is the number of moles in 1000 ml
in 25 ml number of HCl moles - 0.0025 mol
Therefore in 1000 ml - 0.0025 mol /25 x 1000
  concentration of HCl - 0.1 M