The proper solvent choice is crucial for effective recrystallization. What are the features of a good recrystallization solvent?

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Explanation:

Recrystallization is also known as fractional crystallization. And just like fractional distillation, it is used to seperate a mixture of substances (usually a pure substance and its many impurities).

It uses the theory that solubility of solids in solvents increase with temperature.

In normal crystallization, a single solute is contained in the solution and the solution is heated to a high temperature where all of the solute is soluble and then it is gently cooled to enable the solute form as crystals as this temperature drops.

In fractional crystallization or recrystallization, the solutes are dissolved in a 'suitable solvent' at high temperature or dissolved at low temperature and heated to a temperature where all of the solute become soluble. Then, the mixture is cooled slowly such that the solute of interest (the impure solute) crystallizes out first before the other solutes (which are impurities).

So, the properties of a very good recrystallization solvent that can do this job effectively, includes:

- Probably the most important characteristic is that a good recrystallization solvent does not react with the substance to be purified.

- Then, a good recrystallization solvent shouldn't dissolve the substance to be purified at room temperature, but it should dissolve it well at high temperatures or at the solvent's boiling point.

- A good recrystallization solvent should properly dissolve soluble impurities at room temperature.

- A good recrystallization solvent should not dissolve insoluble impurities even at the solvent's boiling point.

- The solvent should not be costly.

- The solvent should be volatile enough so that it can be easily separated from the purified product.

number of molecules / avogadro's number * molecule molecular = mass

explanation:

we define a gram-molecular weight as the mass of one mole of any element or compound. a mole contains avogadro's number of units (molecules) –  

6.022

x

10

23

. so, dividing the number of molecules by avogadro's number gives us the number of moles. multiplying that by the molecular weight gives us the mass. from the periodic table we find the gram-molecular weight of u to be238 and of f to be19. so,  

u

f

6

will have a gram-molecular weight of 352g.