Ch 5: Lab Techniques: Separations and Spectroscopy (Princeton Review)

Question 1

Extraction allows the chemist to separate one substance from a mixture of substances by adding a solvent in which the compound of interest is ______ _______.

Answer 1

highly soluble

p. 112

Question 2

The simplest ______-______ extraction is accomplished when an organic compound is extracted with water. A simple water extraction can remove substances that are highly polar or charged, including inorganic salts, strong acids and bases, and polar, low molecular weight compounds (less than 5 carbons) such as alcohols, amines, and carboxylic acids.

Answer 2

liquid-liquid

p. 112

Question 3

In liquid-liquid extraction, the ratio of the substance’s solubilities in the 2 solvents is called the __________ (or partition) coefficient.

Answer 3

distribution

p. 112

Question 4

All types of chromatography are used to…

Answer 4

…separate mixtures of compounds.
This could be for the purpose of identification, or for purification.

p. 116

Question 5

In TLC, the thin layer of SiO2 coated onto a supporting material like glass acts as the _____ __________ _____.

Answer 5

polar stationary phase

p. 116

Question 6

In TLC, the less polar components have a greater affinity for the solvent than the stationary phase and travel with the mobile solvent at a ______ rate than the more polar components.

Answer 6

faster

p. 116

Question 7

In TLC, often the components are not colored and after running the experiment, need to be detected by some other means. Visualization methods may include shining ___________ light on the plate, placing the thin layer plate in the presence of iodine vapor, or any of many other chemical staining techniques.

Answer 7

ultraviolet

p. 116

Question 8

Assuming you used a polar stationary phase, what does the R_f value signify?

Answer 8

The higher the R_f value the less polar the molecule, or conversely, the lower the R_f value, the more polar the molecule.

p. 117

Question 9

In column (flash) chromatography, in what order can compounds be expected to leave the column and be collected?

Answer 9

In order of polarity, from least polar to most polar

p. 117

Question 10

What is unique about gas chromatography when compared with other chromatography methods?

Answer 10

instead of a solid stationary phase → stationary phase is liquid

instead of a mobile liquid phase → mobile gas phase

p. 123

Question 11

In gas chromatography, separations of compounds occurs based on their _______ _________.

Answer 11

different volatilities

p. 123

Question 12

In gas chromatography, the chart recorder printout enables us to determine the number of components and their ________ amounts.

Answer 12

relative

p. 123

Question 13

In gas chromatography, will the volatile components emerge from the column first or last?

Answer 13

The more volatile components emerge from the column first, while the less volatile components emerge later.

(Remember the example of wind blowing a child vs blowing a sumo wrestler. The more volatile a compound is, the “smaller and lighter” it is.)

p. 124 and Khan Academy video on Gas Chromatography

Question 14

What substances can be used in column chromatography?

Answer 14

silica gel or activated alumina

(from MCAT Self-Prep cards; also p. 117)

Question 15

[Regarding ion exchange chromatography]

If a mixture of proteins is passed through a cation exchange resin (one functionalized with (-) charged groups), those proteins with pI values > pH of the mobile phase will be (+) charged and elute ______ compared to those with pI values below the solution pH.

Answer 15

slowly

p. 119

Question 16

SDS-PAGE can be used to separate most proteins and DNA under __ base pairs. ______ gel can be used for larger proteins and longer DNA.

Answer 16

50

Agarose

Question 17

In gel electrophoresis, the electric field affects all DNA molecules equally, since the mass-to-______ ratios are the same. Instead, we can say that the speed of a DNA molecule under the electric field is _________ proportional to the mass.

Answer 17

charge

inversely

Question 18

In gel electrophoresis, the samples are loaded at the cathode, which is the _________ charged end.

Answer 18

negatively

(cations will migrate toward the cathode, but the DNA is negatively charged, so will move toward the positively charged end)

Question 19

In size exclusion chromatography, the largest molecules move the _______.

Answer 19

fastest

Question 20

_______ _______ is the best technique to use when separating components with very similar boiling points.

Answer 20

Fractional distillation

(MCAT Self-Prep cards)