4. In this experiment, 2.00 g of salicylic acid reacts with an excess amount of acetic anhydride. Calculate the theoretical yield of acetylsalicylic acid for this synthesis.

DAY 1 Preparation of Aspirin Using tap water, set up a boiling water bath in a 400ml beaker. Residual solubility. The solubility of acetylsalicylic acid is 0.25 g per 100 ml of water. Keep track of how much DI water is added to the aspirin during filtration and recrystallization. Mix the starting materials and heat. Using the electronic balance, weigh about 2 g (± 0.01 g) of salicylic acid (caution: this is a skin irritant) in a 125 – ml Erlenmeyer flask. Cover the crystals with 4 – 5 ml of acetic anhydride. (caution: acetic anhydride is a severe eye irritant --- avoid skin and eye contact). Swirl this reaction flask to wet the salicylic acid crystals. Add 5 drops of conc H2SO4 to the mixture and gently heat the reaction flask in a hot water bath (60-70oC) for 5 – 10 minutes. (caution: H2SO4 causes severe skin burns). Set up an ice water bath in a 250ml beaker. Cool to crystallize the aspirin. Remove the reaction flask from the hot water bath and add 10 ml of deionized ice water to decompose any excess acetic anhydride. Chill the solution in an ice bath until crystals of aspirin no longer form, stirring occasionally to decompose residual acetic anhydride. If an “oil” appears instead of a solid, reheat the reaction flask in the hot water bath until the oil disappears and again cool. . Pour the aspirin crystals on to the filter paper. Add 15 ml of “cold” water to the reaction flask, swirl, and chill again. Repeat until the transfer of the crystals to the vacuum filter is complete; maintain the vacuum to dry the crystals as best as possible. Wash the aspirin crystals on the filter paper with 10 ml of ice water. If aspirin forms in the filtrate (the liquid that went through the filter), transfer the filtrate and aspirin to a beaker, chill in an ice bath, and vacuum filter as before, using a new piece of filter paper. If there is no aspirin (solids) in the filtrate, dispose of the filtrate in the sink. Recrystallize the aspirin. Transfer the crystals from the filter paper(s) to a 100 ml beaker. Add ethanol in 5 ml increments until all the crystals are dissolved. You should not need more than 20 ml of ethanol. Warm the mixture in a 60º C water bath (no flame, use a hot plate or a hot water bath). Pour 50 ml of deionized water at about 60º C into the solution. If a solid forms, continue warming until the solid dissolves. Once all solids are dissolved, cover the beaker with a watch glass, remove it from the heat, and set it aside to cool slowly. Set the beaker in an ice bath. If after 10 minutes no crystals are visible, take a clean stirring rod and scratch the bottom of the beaker. This should start the crystallization process. Beautiful needlelike crystals of acetylsalicylic acid form or it may just appear cloudy. Vacuum filter the crystals on a pre-weighed filter paper. Wash the crystals with two 10 ml volumes of ice water. Place the filter paper and aspirin sample on a watch glass and allow them to air-dry. The time for air-drying the sample may require that it be left in your lab drawer until the next laboratory period (Part B). Dispose of the filtrate (liquid that went through the filter) in the waste container. Correct for residual solubility. The solubility of acetylsalicylic acid is 0.25 g per 100 mL of water. That means that when aspirin is in water, 0.25g of it will dissolve for every 100mL. Correcting for this inherent loss of product, calculate the percent yield. For example, if you added a total of 110mL of water to your aspirin during the entire process above, then 0.275 g of aspirin dissolved in the water and was lost through the filter. B. DAY 2 Percent acetylsalicylic acid in the aspirin sample Two trials are to be completed in the analysis of the aspirin. Prepare a clean 250 ml Erlenmeyer flask and determine the mass of two aspirin samples while occupying the balance. Obtain a 50 ml buret from the counter. Prepare the aspirin sample for analysis. Assuming 100% purity of your aspirin sample, calculate the mass of aspirin that requires 20 ml of 0.1 M NaOH to reach the stoichiometric point. YOUR INSTRUCTOR MUST APPROVE these calculations. In a weigh boat, measure the calculated mass (±0.01 g) of the aspirin you have just prepared (or a crushed commercial aspirin tablet) and transfer it to the flask. Add 10 ml of 95% ethanol to try to dissolve the aspirin solids. Then add about 50 ml of deionized water, and swirl to dissolve the aspirin. Add 2 drops of phenolphthalein indicator. Prepare the buret for titration. Condition a clean buret with a standardized 0.1 M NaOH solution. Be sure that no air bubbles are present in the buret tip. After 30 seconds, read and record the volume (±0.02 ml) and the actual molar concentration of the NaOH solution. Titrate the sample. Slowly add the NaOH solution from the buret to the dissolved aspirin sample, swirling the flask after each addition. Initially, add the NaOH solution in 1-2 ml increments. As the endpoint nears, the color fade of the indicator occurs more slowly. Occasionally rinse the wall of the flask with deionized water from your wash bottle. Continue addition of the NaOH titrant until the end point is reached. The endpoint in the titration should be within one-half drop of a faint pink color. The color should persist for 30 seconds. Read and record the final volume of NaOH in the buret. Titrate the remaining aspirin sample.