Question

When asked how much solvent you should use for recrystallization a good rule of thumb is

a) Use a minimal amount
b) Use an excess amount
c) Use a 1:1 ratio with the solute
d) No specific rule

Answer 1

For recrystallization, use the minimal amount of solvent necessary to fully dissolve the solute when it is heated; upon cooling, the solute will precipitate, leading to crystal formation. A 1:1 ratio is not typical, and no universal rule exists because the amount of solvent needed can vary with each solute and solvent combination. The option (A) is correct.

Step-by-step explanation:

When asked how much solvent you should use for recrystallization, a good rule of thumb is to use a minimal amount of solvent that will dissolve the solute completely when hot. This allows for maximum recovery of the solute upon cooling as the compound is less soluble in the solvent at lower temperatures, leading to the formation of crystals. Using an excess amount of solvent may result in a lower yield due to more of the solute remaining dissolved in the larger volume of solvent, even after cooling. The 1:1 ratio is generally not appropriate for recrystallization, and there is no universal specific rule because the amount of solvent needed can vary depending on the particular solute and solvent system.

Steps to perform recrystallization typically include:

1. Dissolving the impure compound in the minimum amount of hot solvent.
2. Cooling the solution to precipitate out the purified compound.
3. Filtering the crystals out of the solution.
4. Drying the purified compound.

Therefore, option (A) is correct.