Lab Midterm

Question 1

What is meant by the term bumping?

Answer 1

sudden and uncontrolled boiling of liquid

Question 2

Identify necessary precautions when carrying out a vacuum filtration with organic materials

Answer 2

inspect the glassware for cracks or chips prior to use

Question 3

What is the purpose of the solvent trap in a vacuum filtration setup?

Answer 3

Keeping the running water from the sink away from the filtration apparatus, and keeping the filtered materials in the filtration apparatus

Question 4

Which solvent should be used to wash the solid material collected from a filtration?

Answer 4

The same solvent as the liquid in the original mixture

Question 5

What modifications to a simple distillation setup can be made in order to increase the speed or efficiency of the separation?

Answer 5

Place the receiving flask in an ice bath.

Question 6

After an organic reaction involving an aqueous solution, the organic solution might be washed with a saturated sodium chloride aqueous solution, known as brine. What is the purpose of the brine wash?

Answer 6

To reduce the amount of water in the organic solution

Question 7

Identify the best support for a separatory funnel.

Answer 7

Iron ring clamped to a ring stand

Question 8

What is the visual indicator that enough of a drying agent, such as anhydrous
MgSO4or CaCl2, has been added to properly dry an organic solution?

Answer 8

The drying agent will move freely like a powder around the solution.

Question 9

After drying an organic solution, what methods can be used to remove the drying agent from solution?

Answer 9

Decanting

Question 10

When working in a fume hood, what is the best position of the hood sash?

Answer 10

As low as possible, no more than halfway up

Question 11

In this organic chemistry lab, why do we determine melting points?

Answer 11

It helps to determine the purity or identity of a sample.

Question 12

What is the “eutectic point”?

Answer 12

The lowest melting point of a mixture of two different compounds

Question 13

Fill in the blank. The melting point of a compound is a _________ property.

Answer 13

physical

Question 14

Why does an impurity broaden a “melting range”?

Answer 14

The impurity disrupts the molecular interactions in different areas of the sample.

Question 15

Why shouldn’t you reuse a sample that was previously melted, even though it has solidified again?

Answer 15

Some of the sample may have decomposed at or near the melting point

Question 16

When selecting a suitable solvent for recrystallization, chemists use what general “rule”?

Answer 16

Like dissolves like

Question 17

Complete the following statement with the correct sequence of words: A suitable recrystallizing solvent is one in which the chemical to be purified is relatively ____________ at low temperature and completely at or near the boiling point of the solvent.”

Answer 17

insoluble, soluble

Question 18

What is the purpose of a hot filtration?

Answer 18

A hot filtration is used to remove insoluble impurities

Question 19

Fill in the blanks with the correct word(s): Slow cooling of a solution will produce crystals.

Answer 19

large, pure

Question 20

When crystals are filtered, why should you wash them with cold solvent?

Answer 20

To remove any remaining solvent, which may contain impurities

Question 21

What does the term “immiscible” mean?

Answer 21

insoluble

Question 22

Why do water and cyclohexame form two layers when mixed?

Answer 22

Water is polar and cyclohexane is nonpolar

Question 23

What is the process of adding brine to a two-phase mixture called?

Answer 23

salting out

Question 24

What is an “emulsion”?

Answer 24

A dispersion of very small droplets of one liquid in another, which are not soluble.

Question 25

What is the surface where thin layer chromatography (TLC) is performed called?

Answer 25

stationary phase

Question 26

What is the TLC plate surface composed of?

Answer 26

silica gel

Question 27

Fill in the blanks with the correct sequence of words: Silica backed TLC plates make excellent stationary phase because it is and very .

Answer 27

stable, polar

Question 28

If the stationary phase of a TLC plate is very polar, this means that polar compounds will migrate
on the plate

Answer 28

a short distance

Question 29

How do you calculate the Rf value?

Answer 29

divide the distance that the spot migrated by the distance the solvent migrated

Question 30

What does a large Rf value (>0.50) indicate?

Answer 30

This implies that the compound interacts more strongly with the mobile phase