Lab Quiz 1

Question 1

How do you calculate the minimum volume of water required to dissolve X grams of a compound

Answer 1

If pure - (ml (from solubility) / g (from solubility)) X amount wanting to be dissolved
If impure - 1. (ml from sol / g given) X given amount(grams) of compound (ex. 0.95 g salicylic)
2. Do the same for the other compound
3. Add them together = ml needed

Question 2

How do you find the amount that will recrystallize after cooling at (given) temp?

Answer 2

Find min ml required to dissolve (ml/g (sol)) x sample g
Then, ml required x solubility at given temp = grams lost
Finally, grams given of compound (ex. 0.95 salicylic) - grams lost = amount that will recrystallize

Question 3

How do you find out if any (given compound) crystals will also form?

Answer 3

Min ml required to dissolve x solubility (g/ml) = max mass of salicylic acid
If it’s more than the mass given, crystals cannot form
If it’s less, crystals will form

Question 4

How is melting point affected if too much sample is used?

Answer 4

Too much sample = uneven heating = less sharp range as the max will increase ( too much sample to be heated)

Question 5

How would melting point range be affected if the sample is contaminated?

Answer 5

It will decrease as the impurities are embedded in the lattice - intermolecular forces are weakened

Question 6

How would the melting point be affected if heated too rapidly to the literature MP?

Answer 6

It will pass the MP quickly, making the measured range higher than expected

Question 7

What does successful recrystallization depend on?

Answer 7

A large difference between the solubility of a compound in hot solvent and in the same solvent cold

Question 8

What technique is typically used in recrystallization?

Answer 8

Dissolve the solid in hot solvent then cool the solution slowly to promote crystallization
Solid + solvent +/- heat - solution

Question 9

What happens to the impurities in a single compound recrystallization?

Answer 9

They either remain dissolved in the solvent/filtrate or remain insoluble and are removed by filtration

Question 10

What is included in the crystal lattice if done properly?

Answer 10

Crystals are grown slowly and only crystallized produce is included in their lattice

Question 11

What is precipitation? Why do we prefer recrystallization?

Answer 11

Precipitation is when crystals are formed rapidly and incorporate impurities in the lattice. We prefer recrystallization because it gives us a purer product

Question 12

When are impurities incorporated into the lattice? What does rapid cooling cause?

Answer 12

Impurities are incorporated into the lattice when the equation is shifted by sudden rapid cooling (traps impurities)
Rapid cooling causes smaller crystals with a larger combined SA - hard to wash clean

Question 13

How should a solid be chosen for single compound recrystallization?

Answer 13

It should be insoluble in the solvent at low temperatures and highly soluble in the solvent at elevated temperatures

Question 14

What is the criteria for selecting an appropriate solvent (single)?

Answer 14

1. Should have a moderate boiling point (50-130) so that the solvent is easily removed from the crystals
2. The boiling point should be below the melting point of the compound being crystallized (prevents oiling out)
3. Low flammability if possible
4. Should not react irreversibly with the solid, and impurities should be insoluble in the solvent (hot or cold)

Question 15

Why are solvents picked to be of a similar chemical nature to the solid?

Answer 15

Like dissolves like rule - salicylic acid has a non polar aromatic ring and a polar carbonyl and hydroxyl group - soluble in water

Question 16

What are some common polar solvents?

Answer 16

Water, methanol, ethanol , acetone

Question 17

What are some common non polar solvents?

Answer 17

Pentane, hexane, benzene, toulnene (?)

Question 18

What is oiling out? Why does it happen?

Answer 18

When the crude compound separates as an oil instead of dissolving in the solvent. Happens if MP is less than BP - solid with a low MP is being recrystallized
The impurities will distribute between the oil and solvent phases and become an amorphous solid

Question 19

How can oiling out be prevented?

Answer 19

Keep the Temperature of the solvent lower than the melting point of the solid, avoid heating solvent to the B.P.

Question 20

What is percent recovery? Why is it less than 100? How is it calculated?

Answer 20

The yield, is less than 100% bc impurities remain dissolved in the solution, and a small amount of solid is dissolved in the filtrate.

Question 21

What is melting point used to assess? How is it assessed?

Answer 21

Used to assess purity, done by heating gradually -energy of the molecules destroys the lattice- the solid becomes liquid

Question 22

How is melting point determined? What physical changes happen before melting?

Answer 22

First drop of liquid - last crystal becoming liquid

Partial decomposition, color change, sintering (shrinking or softening)

Question 23

When is a compound pure based on its melting point

Answer 23

Narrow range (1-2 is pure, 2-3 is okay, 3+ is impure) 
MP should be close to the literature MP

Question 24

What four things might affect melting point?

Answer 24

Amount of sample, size of crystals, apparatus used, rate of heating

Question 25

How can you correct the thermometer?

Answer 25

At the manufacturer, using a calibration graph, using an MP standard

Question 26

Gravity vs vacuum filtration

Answer 26

Hot gravity - solid separated from liquid at the boiling point of the liquid
Cold gravity- solid separated from liquid at room temperature
Vacuum - solid separated from liquid at an air pressure lower than atmospheric - collect solids/crystals

Question 27

When is a mixed solvent recrystallization used?

Answer 27

When a solvent with the desired solubility characteristics cannot be found

Question 28

What is required of the compound and solvents?

Answer 28

Compound purified must be soluble in one solvent and insoluble in the other at all temperatures!! The solvents should be Miscible

Question 29

List some common solvent pairs

Answer 29

Methanol water, ethanol water, acetic acid water, acetone water, ether petroleum ether, toluene ligroin, dioxane water

Question 30

How is a mixed solvent recrystallization performed?

Answer 30

1. Dissolve in the min volume of boiling solvent in which it is soluble
2. The second solvent (in which solid is insoluble) is heated close to the BP then added dropwise into the boiling solution until the mixture is cloudy
3. Clear the mixture by adding a small amount of the first solvent
4. The solution will be saturated and should form crystals upon cooling

Question 31

Why shouldn’t you add an excess of The second solvent or cool too rapidly?

Answer 31

Oiling out- if oiling out, reheat the solution and add more of the soluble solvent

Question 32

What is a mixed melting point test used for? How does it work? What do the results mean?

Answer 32

Provide evidence in identifying a compound
- measure the MP of the unknown mixture and measure a known compound with a similar MP (suspected to be unknown), and a mix of the two
If it is the unknown - the MP won’t be depressed, same MP as each other
If different - depression and broadening of the range
Requires a standard of the compound

Question 33

How do impurities affect the mixed solvent MP test?

Answer 33

They DO NOT depress it

They can be solid or liquid so it is important to dry it well

Question 34

Group 1 compounds and MPS

Answer 34

Benzil (94-95), methyl 4-nitrobenzoate (94-96)

Question 35

Group 2 compounds and MPs

Answer 35

3-nitroaniline (112-114), 2-methyl- 1,4 -naphthoquinone (105-107)

Question 36

Group 3 compounds and MPS

Answer 36

4-methylacetaniliae (144-151), 4-nitroaniline (149-151)