(a) Failing to stir or shake the dichloromethane and Panacetin mixture long enough would lead to a variable amount of undissolved Panacetin, which could be left on the filter paper during gravity filtration, thereby increasing the mass of sucrose. This could also lower the percent compositions of aspirin and the unknown because less Panacetin would be present than if all of the Panacetin was dissolved in the dichloromethane.
(b) Failing to mix the aqueous and organic layers thoroughly during the NaHCO3 extraction would lead to little to no production of aspirin precipitate. The aqueous layer contains sodium hydroxide and water, whereas the organic layer contains the dichloromethane, aspirin, and the unknown. If the HCl has nothing to react with in the aqueous layer following mixing (i.e., sodium acetylsalicylate), then little to no solid aspirin will be produced. Furthermore, more unknown would be produced than expected because aspirin would precipitate with the unknown during distillation.
(c) Mistakenly extracting the dichloromethane solution with 5% HCl rather than 5% NaHCO3 would make the solution impure. It would likely not form an aqueous and organic layer, thus making it impossible to separate aspirin and the unknown