Part II Read through the revised set of instructions (you may have to rotate the

Question

Part II

Read through the revised set of instructions (you may have to rotate the file). Again, imagine yourself using these in a laboratory setting. How successful is the revision?

Structure and Organization 249 Chapter 10 Instructions, Procedures, and Policies CASE DOCUMENT 10-1B Recrystallization Step-by-step instructions for carefully and correctly performing Conducting the Experiment This experiment requires five steps: I. Dissolving the compound, 2. Using the balance, 3. Setting up the hot vacuum filtration apparatus, 4. Recrystallizing the acetanilide, and, 5. Cleaning up following the procedure Recrystallization. Recrystallization is a technique used to separate and purify organic com- pounds from liquid to solid matter. 1. Dissolving the compound Before starting this experiment, think safety! These chemicals could be haz- ardous to your skin and eyes. 1. Using a graduated cylinder, measure 75 mL of water and add it to a 250 mL beaker turn the red knob at the side of your desk to begin heating. impure acetanilide (Impure acetanilide can be found in a bottle at the front 2. Set the beaker on the cylindrical steam bath on your laboratory desk and For Safety: .Keep your safety goggles on at all times. Tie back long hair Wear closed-toe shoes. Wear gloves if needed. Do not consume any food, drinks, or chemicals during lab. Do not wear loose clothing. 3. While your water is heating, use the balance to weigh approximately 1.5gof counter). Use a watch glass to accurately weigh the impure acetanilide. 2. Using the balance . Obtain a watch glass from your instruc Equipment Required: Check off items Das you collect these before you begirn. tor and set it on the balance. Figure 1 is an example of a watch glass. 2. Tare the balance by pushing the "tare" Crucible Tongs 250-mL Beaker 125-mL Erlenmeyer Flask Stirring Rod Boiling Chip Steam Bath button so that the watch glass has a mass of 0.0 grams. OFilter Flask Buchner Funnel Black Tube Filter Paper 3. Then using a spatula, carefully add Figure 1-Watch Glass impure acetanilide onto the watch glass until the balance reads approximately 1.5 grams. Then add the impure acetanilide to the 125-mL Erlenmeyer flask. O Watch Glass OClamp Rubber FilterVac 4. Break up the clumps of acetanilide with a stirring rod and add a black 5. Add approximately 25 mL of the heated water from the beaker into the 6. Using crucible tongs, swirl the flask occasionally to prevent clumping. boiling chip (found at the front counter) to the flask. Chemicals used: flask containing the impure acetanilide. Water Acetanilide' Crucible tongs will protect you from burning your hands. Never pick up hot objects with your hands. Organic-relating to chemical compounds containing carbon, especially hydrocarbons Acetanilide-a white, crystalline, odorless, organic powder, CaHsNO, produced by the action of glacial acetic acid on aniline, used chiefly in organic synthesis and formerly in the treatment of fever Tare-to obtain the weight of a container or wrapper that is deducted from the gross weight to obtain net weight. Crucible-metal or refractory material employed for heating substances to high temperatures and headache. continued

Explanation / Answer

On going through the above experiment setup, it all seems absolutely alright. Although, in order to have successful recrystallisation, the hot solution of impure acetanilide must be a highly saturated solution. Only then will the amount of recrystallized pure acetanilide be significant, for you to filter, using the hot vaccum filtration process.

To prepare a highly saturated solution of impure acetanilide, the hot solution must be infused with as much acetanilide and it can possibly dissolve or the other alternative would be to use minimum solvent ( here the solvent being water). The recrystallisation would be successfull if the solution is saturated and yet the solute (acetanilide) dissolves completely in the hot sovlent (water).

This is followed by cooling of the solution. The process of cooling should not be instantaneous but a gradual process so that the crystals would form perfectly, just according to its lattice. If the solution is cooled immediatly, in would precipitate instead of recrysallizing.

"An important fact to be remembered here is that: The best result of recrystallisation is obtained only when the solute is completely soluble in the hot solvent and completely insoluble in the same solvent when it is cooled."

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