please explain why the molar concentration is considered as [OH-] instead of [H3O+]. When is the concentration considered as H3O+? Please explain thoroughly, i will give a thumbs up :)

Calculate the pH during the titration of 50.00mL of 0.0500MNaOH with 0.1000MHCl at 25C after the addition of the following volumes of reagent: (a) 24.50mL, (b) 25.00mL, (c) 25.50mL. Solution (a) At 24.50mL added, [H3O+]is very small and cannot be computed from stoichiometric considerations but can be obtained from [OH]: [OH][H3O+]pH=cNaOH=totalvolumeofsolutionoriginalno.mmolNaOHno.mmolHCladded=50.00+24.5050.000.050024.500.1000=6.71104M=Kw/(6.71104)=1.001014/(6.71104)=1.491011M=log(1.491011)=10.83