Expert Q&A; Done A student prepared aspirin by adding 2.005 grams of salicylic a

Question

Expert Q&A; Done A student prepared aspirin by adding 2.005 grams of salicylic acid to 4.50 mL of acetic anhydride. The reaction took place in the presence of the catalyst, phosphoric acid. After heating at 75 C for 15 minutes, the reaction was complete. The student filtered the aspirin and dried the crystals. The final mass of aspirin was 2 305 grams. Complete the following calculations . Determine the molar masses of a acetic anhydride CaH.O e and aspirin, CH.Os How many moles of salicylic acid were added to the reaction mixture? How many moles of aspirin could be produced from the salicylic acid What was the mass of acetic anhydride added to the reaction mixture (density How many moles of acetic ashydride were added? 1.08E-L12 How many moles of aspirin could be produced tom this u Refer to questions 3 and 5. How many moles of aspirin can actually be peepared What is the theoretical yield i·grams oraspem What is the percentage yield of aspiin? A student fond ort of acetil·shydride? 6. 7, 9. the percentage yield esceeded 100% Explain what could have happened to the sample.

Explanation / Answer

To determine the answers for the questions given, first the molar masses of salicylic acid, acetic anhydride and aspirin are required. These are 138.121g/mol, 102.09g/mol and 180.157g/mol respectively.

Now, the no.of moles of a substance is calculated from molar mass and mass taken as no.of moles = weight of compound/molar mass of compound. This gives no.of moles of salicylic acid as 2.005/138.121 = 1.4516x10-2moles and no.of moles of aspirin is 2.305/180.157 = 1.2794x10-2moles. To find the mass of acetic anhydride, using its volume and its density, the mass is calculated as mass = density x volume = 1.08 x 4.5 = 4.86g which gives no.of moles of the anhydride as 4.86/102.09 = 4.7605x10-2moles.

The theoretical yield of aspirin is calculated as mass of aspirin equal to the number of moles of the limiting reagent in the reaction. Here, comparing the number of moles of the two reactants it can be determined that salicylic acid is present in lower moles than acetic anhydride. Therefore, when all of salicylic acid has finished getting acetylated, there would still be a certain quantity of acetic anhydride left making the acid the limiting reagent implying that the number of moles of aspirin that should have been produced from this reaction should be 1.4516x10-2moles which will give 1.4516x10-2 x 180.157 = 2.6151g of aspirin. However it can be seen that a lesser quantity of aspirin than theoretical has been prepared. If the acetic anhydride is the limiting reagent, 4.7605x10-2moles of aspirin should have produced which is equal to 4.7605x10-2 x 180.157 = 8.5673g of aspirin.

Thus, the percentage yield has to be calculated now and that is given by (Experimental yield/Theoretical yield)x100 = (2.305/2.6151) x 100 = 88..142%.

In the rare case a yield of greater than 100% is obtained, from the protocol for the reaction given, the error can lie in improper weighing resulting excess reagents or if that has been done correctly, excess yield might show anomalously if the precipitate is not dried properly resulting in the mass of water or solvent adding to that of aspirin's mass without the knowledge of the student.

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