Organic chemistry lab: recrystalization & melting point 1. Give two examples of

Question

Organic chemistry lab: recrystalization & melting point

1. Give two examples of an advantage of using a Craig tube in a crystalization and two disadvantages.

2. Sometimes when performing a crystalization, one solvent alone will not work and you have to use a solvent-pair. Will the solvent pair ether and water work? Why or why not?

3. You recrystallize 500mg of a solid material and obtain 200mg. What is your percent recovery? If you concentrate (evaporate) some of the mother liquor and obtain a further 50mg, what is your total percent recovery?

4. Sometimes during a crystallization, crystals need to be coaxed out of solution. Describe two methods that can achieve this.

5. When is the melting range of a substance recorded?

6. Why is it suggested you use weighing paper or a watch glass instead of filter paper on which to allow a sample to dry? Please give two specific examples.

Explanation / Answer

- Give two examples of an advantage of using a Craig tube in a crystalization and two disadvantages.

Advantages examples:

Allows to minimize the number of transfers of the solid material having as a result a better yield of crystals.

The second advantage is that the Craig tube is very efficient and faster to do regarding the separation of crystals from the mother liquor.

Disadvantages examples:

One of the disadvantages is the cost and fragility (Craig tubes made of glass can cost as much as 15$).

Another disadvantage is that it can not hold large volumes, since it is used for small-scale recrystallization.

- Sometimes when performing a crystalization, one solvent alone will not work and you have to use a solvent-pair. Will the solvent pair ether and water work? Why or why not?

This combination, as a solvent-pair, would not work because ether and water are not miscible with each other. One thing that solvent pairs requires is that the two solvents must be miscible with each other.

- You recrystallize 500mg of a solid material and obtain 200mg. What is your percent recovery? If you concentrate (evaporate) some of the mother liquor and obtain a further 50mg, what is your total percent recovery?

Here we need to use the formula for percent yield, which is:

Percent yield = actual yield / theorethical yield x 100

Calculating first percent recovery:

Actual yield = 200 mg

Theorethical yield = 500 mg

Percent yield = 200 mg / 500 mg x 100 = 40 %

Calculating TOTAL percent recovery:

Actual yield = 200 mg + 50 mg (since this is the last recovery obtained from the mother liquor)

Theorethical yield = 500 mg

Percent yield = (200 mg + 50 mg) / 500 mg x 100 = 50 %

- Sometimes during a crystallization, crystals need to be coaxed out of solution. Describe two methods that can achieve this.

For different experiments where recrystallization is required, an ice bath (ice and water bath) is applied to coaxed out crystals out of a solution.

Sometimes when the crystal have not formed after a period scratching the glass of the container with a rod will induce crystallization.

- When is the melting range of a substance recorded?

When we want to determine the melting point of a substance we record the range of temperatures when the first crystal starts to melt until the temperature of the last crystal disappears.

The meting range is recorded when we want to determine the level of purity of a substance (an impure compound gives a wider range of temperature). The melting point helps to identify unknown substances (for example, a pure solid melts reproducibly over a narrow range of temperature).

- Why is it suggested you use weighing paper or a watch glass instead of filter paper on which to allow a sample to dry? Please give two specific examples.

Filter paper is porous and some of your sample may stay on it, and this will give you a less amount of your sample.

Filter paper can absorb water, changing the mass of the filter and changing the mass of your sample.

This kind of things do not occur in weighing paper or watch glass.

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