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ochem lab > recrystallization > Flashcards

recrystallization Flashcards

1
Q

What technique will you use to purify your pharmaceutical acid?

A

recrystallization

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2
Q

What solvent will you use to do recrystallization?

A

water

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3
Q

What melting point range will you be looking for to confirm that your sample is pure?

A

1-2 degrees C

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4
Q

How does temperature generally affect solubility, specifically the solubility constant S.
Solubility (S) decreases at higher temperature
Solubility (S) increases at higher temperature
Solubility (S) has not effect on temperature

A

Solubility (S) increases at higher temperature

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5
Q

Why do you need to heat the beaker that contains the solution of the acid you are recrystallizing?
to melt the acid
to do a chemical reaction
to increase the solubility of acid in water

A

to increase the solubility of acid in water

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6
Q

Why do you need to heat the beaker that contains the solution of the acid you are recrystallizing?
to melt the acid
to do a chemical reaction
to increase the solubility of acid in water

A

to increase the solubility of acid in water

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7
Q

According to the procedure, how much solvent will you use to dissolve the acid when performing your recrystallization?
as little as possible
as much as possible
exactly 3 drops

A

as little as possible

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8
Q

What technique is used for identification of the unknown acid in this experiment?
infrared spectroscopy
mixed melting point
mixed boiling point
recrystallization

A

mixed melting point

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9
Q

How will you cool the solution below room temperature to maximize the quantity of crystals of pure acid that will form?
Pour the solution on the ice bath
Place the solution on an ice bath
Blow air to the solution

A

place the solution on an ice bath

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10
Q

The names of the two acids that could potentially be your unknown acid are:
benzoic acid and salicylic acid
acetic acid and benzoic acid
salicylic acid and acetic acid
acetylsalicylic acid and salicylic acid

A

benzoic acid and salicylic acid

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11
Q

When you calculate a % recovery you will usually find that it is smaller than 100%. Which of the following are possible reasons for this phenomenon?
The impurity has been separated out
Mechanical losses (e.g. during transfers)
Some of the compound is still in solution
All of the above are possible reasons

A

All of the above are possible reasons

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12
Q

Suppose, after recrystallization and subsequent filtration of your pure sample, you don’t allow enough time for it to dry before measuring its weight. What do you expect your % recovery would be?

<100%
>100%
Cannot be determined. Need more information.

A

> 100%

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13
Q

Suppose, after recrystallization and subsequent filtration of a pure sample, a student doesn’t allow enough time for it to dry before performing its melting point analysis. So, the sample is still wet with the solvent. The student observes that the melting range of the sample is both broader and lower than what is expected.

What is a possible reason for this observation?

The recrystallization was a failure
The solvent is acting as an impurity
The DigiMelt wasn’t pre-heated to the appropriate temperature
The DigiMelt instrument needs re-calibration

A

The solvent is acting as an impurity

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14
Q

In your pharmaceutical formulation, benzoic acid is mixed with salicylic acid in a roughly 2:1 ratio.

If you measured the melting point of this mixture, would the benzoic acid melt at a temperature that is the same as pure benzoic acid, or would it melt at a temperature lower than that of pure benzoic acid or at a temperature higher than that of pure benzoic acid?

higher
lower
same

A

lower

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15
Q

What is the formula for describing the available thermal energy at any temperature?
Thermal Energy = mcdT
Thermal Energy = R log T
Thermal Energy = RT
All of the above

A

Thermal Energy = RT

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16
Q

Ideally, where are the impurities at the end of a recrystallization procedure?
In the solid crystals
In the solvent
On the filter paper
On the dry plate

A

In the solvent

17
Q

are hydrogen bonds dashed or solid lines

A

dashed

18
Q

are covalent bonds solid or dashed lines

A

solid

19
Q

Recrystallization “works” by allowing crystals of the desired compound to grow in a cooling solution, while excluding impurity molecules from the crystal because they will not “fit” properly in the highly ordered crystal. If any impurity molecules did manage to get into one of the forming crystals, what would be the effect on the melting point of that particular crystal?
HIGHER and NARROW the melting range
LOWER and NARROW the melting range
LOWER and BROADEN the melting range
HIGHER and BROADEN the melting range

A

LOWER and BROADEN the melting range

ochem lab (9 decks)

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