The post-lab questions provided are SIMILAR to questions you may encounter in your lab manual. These questions will provide you with a step-by-step guide on how we approached the problem. You can use this guide and the data you recorded to help answer the questions. Any given examples with values will be with mock values. Use of these values will almost assuredly give you nonsensical calculations. They are only meant to illustrate the logic.

question 1

You are asked to calculate the percent yield of aspirin, give what the value should be (theoretical yield), and explain why your value (actual yield) may vary from this

From the above reaction we know the amount of salicylic acid is a 1 to 1 (1:1) ratio with the amount of aspirin. This means every mole of salicylic acid used will yield the same number of moles of aspirin. Therefore, the first step is to calculate the number of moles of salicylic acid used. Note: you are told that acetic anyhydride is in excess and salicylic acid is the limiting reagent

1. Mass of Salicylic Acid Used (mock value) = 276.24 g

2. Molar Mass of Salicylic Acid = 138.12 g

3. Moles of Salicylic Acid = (1)/(2) = 276.24/138.12 = 2.000 moles

To then convert the number moles of Aspirin to the expected yield in grams, we multiply the moles by the molar mass of asprin

1. Moles of Salicylic Acid = Moles of Aspirin —> 2 moles of Salicylic Acid makes 2 moles of Asprin

2. Molar Mass of Aspirin = 180.16 g

3. Theoretical yield of Aspirin (g) = (2 moles)(molar mass of aspirin) = 2.000(180.16) = 360.3 grams

Percent Yield Calculation (remember significant figures)

Your actual yield will be the amount aspirin you weighed after it was filtered and dried in lab. Using the theoretical and actual you can calculate percent yield.

1. Actual Yield of Aspirin in lab after filtered/dried (mock value) = 370 g

2. Mass of Theoretical Yield of Aspirin (mock value) = 360.3 g

3. Percent Yield = (Actual ÷ Theoretical) x 100 = (370 ÷ 360.3) x 100 = 103 %

Your value may vary from the theoretical value for a number of reasons. If it is above 100%, it may be because your aspirin was not completely dried. If it is below 100%, it may be because of sample loss when transferring to the weigh paper.