We are given equal amounts of ammonia and ammonium chloride which are mixed to be made a buffer at a pH of 9.25. To strengthen the buffering capacity of ammonia and ammonium chloride, we need to add the amount of ammonium chloride, it should be higher than ammonia with a certain ratio since not all ammonia will react with NH3Cl, so if we add NH3Cl, the ammonia left will be able to react with NH3Cl to complete the reaction. This will strengthen the buffering capacity of the mixture and decrease the pH a bit.
