Synthesis of Acetylsalicylic acid: (a) Place approx. 1.4 g (±0.01 g) of salicylic acid (Caution: this is a skin irritant) in a dry 100 mL conical flask. Record the exact mass to three decimal places. (b) Add 3.3 mL of acetic anhydride (Caution: acetic anhydride is a severe eye irritant – avoid skin and eye contact) in such a way as to wash down any material adhering to the walls of the flask, and then add 1 drop of concentrated sulfuric acid (Caution: sulfuric acid causes severe skin burns). (c) Heat flask for 15 min. on a steam bath and without allowing to cool, add 2 mL of water and allow the flask to stand at room temperature until crystallisation begins. The crystallisation can be hastened by occasionally scratching the walls of the flask at the surface of the solution with a glass stirring rod. When the crystals begin to appear, place the flask in an ice bath, add 10-15 mL of cold water and chill thoroughly until crystallisation is complete. (d) Collect the product on a Buchner funnel and wash the crystals thoroughly with ice-cold water (10 mL per wash, 3 washes). (e) Place the crystals on a watch glass, labelled underneath with your name. (f) Dry in the oven for ~10 minutes. (g) Calculate the yield (%) and measure the melting point. (h) Place the product in a labelled vial and hand it in. (i) Measure the melting point of pure acetylsalicylic acid (Aspirin). This may be done in the following lab if you do not have enough time. NOTE: In this virtual lab, you will be measuring the melting point later in this lesson. Record all your results and calculations in your workbook and answer the questions. Calculation of yields Your goal for this section is to calculate the percentage yield. This relates how much product you physically created compared to how much product was theoretically possible. Chemical reactions are never 100% efficient so the amount of product physically created is always less than what is indicated by a balanced equation. You may find it useful to review your notes from trimester one in order to do the calculations. In brief the steps you take to arrive at the yield are: Calculate the number of moles of reactant (salicylic acid) you have from the mass and molecular weight of salicylic acid. Use the balanced equation to determine how many moles of product (acetylsalicylic acid) you should theoretically obtain. Convert the moles of acetylsalicylic acid to grams of acetylsalicylic acid. Calculate the per cent yield.
Synthesis of Acetylsalicylic acid:
(a) Place approx. 1.4 g (±0.01 g) of salicylic acid (Caution: this is a skin irritant) in a dry 100 mL conical flask. Record the exact mass to three decimal places.
(b) Add 3.3 mL of acetic anhydride (Caution: acetic anhydride is a severe eye irritant – avoid skin and eye contact) in such a way as to wash down any material adhering to the walls of the flask, and then add 1 drop of concentrated sulfuric acid (Caution: sulfuric acid causes severe skin burns).
(c) Heat flask for 15 min. on a steam bath and without allowing to cool, add 2 mL of water and allow the flask to stand at room temperature until crystallisation begins. The crystallisation can be hastened by occasionally scratching the walls of the flask at the surface of the solution with a glass stirring rod. When the crystals begin to appear, place the flask in an ice bath, add 10-15 mL of cold water and chill thoroughly until crystallisation is complete.
(d) Collect the product on a Buchner funnel and wash the crystals thoroughly with ice-cold water (10 mL per wash, 3 washes).
(e) Place the crystals on a watch glass, labelled underneath with your name.
(f) Dry in the oven for ~10 minutes.
(g) Calculate the yield (%) and measure the melting point.
(h) Place the product in a labelled vial and hand it in.
(i) Measure the melting point of pure acetylsalicylic acid (Aspirin). This may be done in the following lab if you do not have enough time. NOTE: In this virtual lab, you will be measuring the melting point later in this lesson.
Record all your results and calculations in your workbook and answer the questions.
Calculation of yields
Your goal for this section is to calculate the percentage yield. This relates how much product you physically created compared to how much product was theoretically possible.
In brief the steps you take to arrive at the yield are:
- Calculate the number of moles of reactant (salicylic acid) you have from the mass and molecular weight of salicylic acid.
- Use the balanced equation to determine how many moles of product (acetylsalicylic acid) you should theoretically obtain.
- Convert the moles of acetylsalicylic acid to grams of acetylsalicylic acid.
- Calculate the per cent yield.
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