o HCl is a strong acid, whereas NH₄OH is a weak base, their reaction :

HCl + NH₄OH = NH₄Cl + H₂O

Hence all of 0.05 mol dm^-3 of HCl will be neutralized by NH₄OH and after completion of neutralization there will be some amount of NH₄OH left (concentration 0.1 mol dm^-3), so that the reaction mixture results in NH₄OH + NH₄Cl which will form a buffer solution.