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