The active agent in Aspirin is the salicylic acid, it is not an impurity.
1 mole NaOH has mass 40g. 1 g will have 0.025 moles in it.
the total volume when the HCl has gone in is 72.7 cm3
the number of moles HCl used in 22.7 cm3 is n=v/1000 x concn number =22.7/1000 x 0.5 = 0.0114
ASA has one carboxylic acid group.
there must be a TOTAL of 0.025 moles acid to perfectly neutralise 1 g of NaOH. 0.0114 gets supplied by the HCl, so the rest of it is the ASA. This means there are 0.0137 moles of ASA.
ASA has a formula of C7H6O3. So this is a MM of (12 x 7) + (1 x 6) + (16 x 3) = 138
so 1 mole has a mass of 138 g, so 0.0137 moles has a mass of 1.89 g which is MORE than the mass of impure aspirin.
all I can assume is that the aspirin was not broken down fully and so only part of it actually neutralised the NaOH