Question

Construct a solubility curve similar to one in Figure 11.2 for acetanilide and water. The solubility of acetanilide in water at 0⁰C is 0.0052 g/ mL water; and at 100 ⁰C is 0.055 g.

Naphthalene, an organic compound is used as fumigant, mothball. Fumigant uses of naphthalene include use in soil as a fumigant pesticide, in attics spaces to repel animals and insects, and in museum storage-drawers and cupboards to protect the contents from attack by insect pests. 100.00 mg of an impure form of naphthalene contains 20% impurities is purified using methylene chloride as a solvent. Calculate the % yield if 74.68 mg naphthalene is recovered. The solubility of naphthalene in methylene chloride at its boiling point is 0.0842 mg/mL; and at its freezing point is 0.0195 mg/mL.

What is the purpose of using a boiling stone when boiling a liquid?

Why is minimum amount of boiling (hot) crystalizing solvent used when dissolving a solid compound?

What do you do if crystals do not form even after leaving them on the bench for 30 minutes?

Why do you wash the purified crystals with cold solvent?

Sketch an approximate drawing of a Craig Tube apparatus placed in a centrifuge tube and ready to be centrifuged.

A good solvent for crystallization has to be: 1. Nontoxic. 2. Nonreactive. 3. Nonvolatile. 4. Inexpensive. List three other characterization of a good crystalizing solvent.

Comment

Answer 1

1.

Solubility of acetanilide in water increases with temperature . So the slope of the curve will be positive .

At T = 0 degreeC solubility of acetanilide in water = 0.0052 g/mL and at 100 degreeC = 0.055 g/mL

So solubility curve will be like an curve having exponenetial growth where we can calculate the slope of each part which will be 0.055 - 0.0052 / 100 = 0.000498 . So curve will have exponential growth with slope of 0.000498 g/mL-degreeC

2.

100 mg of impure napthalene contains 20% impurities. Percentage of napthalene in impure sample = 100 - 20 = 80%

So,

amount of napthalene in impure sample = (80 / 100) * 100 mg = 80 mg

So,

Theoretical yield = 80 mg

Actual yield = 74.68 mg

So,

% yield = (Actual yield / theoretical yield) * 100% = (74.68 / 80) * 100%

% yield = 93.35%

3.

A boiling stone is generally used to speed up the boiling process because when a boiling stone is heated then the air which is trapped inside the boiling stone expands and escapes the stone . If this gas comes in contact with a heated liquid then the boiling of the liquid will be much quicker . So for this reason the boiling stone is used.

4.

When dissolving a solid compound , minimum amount of boiling (hot) crystalizing solvent used is used because if too much solvent is used then the recovery of the sample is decreased . So in order to recover an effective amount of sample the boiling crystalizing solvent used is in minimum amount.

5.

If crystallization does not occur even after leaving the solution on the bench for 30 minutes then we can induce crystallization by cooling the solution in an ice bath or by scratching the vessel wall with a glass stirring rod.

6.

The purified crystals are washed with cold solvent in order to remove the impurities if there are any from the purified crystals.

7.

Craig tube apparatus consists of a inner plud made of glass along which is placed in a outer tube where the crystals will be placed inside a centrifuge tube .

8.

Three other characteristics of a good solvent for crystallization are :

(a) The boiling point of solvent should be lower than melting point of the compound

(b) The compound should be insoluble in the solvent at room temperature.

(c) The compound should be completely soluble in hot boiling solvent.