Determine the number of moles of sodium hydroxide used to dissolve the aspirin tablet. (This will be the same in all three cases.)
From your titration data, determine the number of moles of hydrochloric acid added, and hence determine the number of moles of sodium hydroxide that did not react with the aspirin.
From the quantities determined so far, calculate the number of moles of sodium hydroxide that reacted with the acetylsalicylic acid in the aspirin.
Remembering that 1 mole of acetylsalicylic acid reacts with 2 moles of sodium hydroxide, calculate the number of moles of acetylsalicylic acid in the aspirin tablet.
Calculate the mass of acetylsalicylic acid in the aspirin tablet.
Determine the percentage of acetylsalicylic acid in the aspirin tablet.