MCAT Laboratory Techniques

Question 1

” like dissolves like”

Answer 1

Important in separation techniques in which polar compounds are attracted to polar solvents and nonpolar compounds are attracted to nonpolar solvents.

Question 2

Miscibility

Answer 2

The degree to which a solute can dissolve in a solvent.

Question 3

Aqueous solvents v. Organic solvents

Answer 3

Aqueous solvents are polar and attracted to solutes capable of hydrogen accepting and donating.

Organic solvents can be polar or nonpolar.

Question 4

What are some of the low- polarity and high- polarity organic solvents?

Answer 4

low - polarity- esters, ethers, halogenated hydrocarbons, aromatic solvents, hydrocarbons.

high- polarity - short chain alcohols, THF, acetic acid, polar aprotic solvents.

Question 5

Purification, what are the two types?

Answer 5

A separation method that results in a purified solid being removed from the mixture.

• Physical purification - changes affecting physical properties of mixtures to allow solidification out of mixture. (i.e. changing temperature)
• Chemical purification- chemical changes that allows solidification out of solution. ( i.e. acid- base reactions)

Question 6

Trituration

Answer 6

When we pour a solvent on a solid that dissolves the impurities and leaving the solid undissolved, we can isolate the solid via filtration.

Question 7

Recrystallization

Answer 7

A boiling solvent is poured onto solid and then allowing it to cool. The purified sample cools out of solution forming a solvent and impurities stay dissolved in solvent.

Some of the desired product is in solvent alongside the impurities this is called the mother liquor. To get that desired product back we can dissolve the solvent.

Question 8

Mixed solvent recrystallization

Answer 8

We first add a hot solvent with high solubility which dissolves the solid. We then add the hot solvent with low solubility. When we reach the cloud point the purified solid is formed.

Question 9

Filtration. What are the two types?

Answer 9

Separation process in which we separate a solid ( precipitate) from the solvent ( filtrate).

• Gravity filtration- when we use a pourous filter and pour the solid- solvent mixture through and allowing gravity to separate precipitate ( stays in filter) and filtrate ( passes through).
• vacuum filtration - we use a vacuum to suck the filtrate through and separate it from the precipitate. In this technique we use the buchner funnel.

We can pour a cold solvent over the filtrate to remove further impurities ,this is called a wash.

The desired product can be in the solute, solvent, or both.

Question 10

liquid - liquid separation

Answer 10

Substances are dissolved in polar solvent and organic solvent and these solvents are separated due to immiscibility. Organic solvent is less dense and rises to the top.

Question 11

Partition coefficient

Answer 11

The solubility of a substance in an organic solvent compared to aqueous solvent.

P = [A] organic/ [A] water

Larger partition coefficient means higher solubility in organic solvent.

Smaller partition coefficient means higher solubility in aqueous solvent.

Question 12

Extraction v. Wash

Answer 12

Extraction is when we add a solvent to the mixture and the desired product dissolves into the added solvent.

Wash is when we add a solvent and the undesired product dissolves into the added solvent.

Question 13

How does liquid- liquid extractions work?

Answer 13

Mix two immiscible liquids into a separatory funnel and allow them to separate ( partitioning). Components of mixture that are nonpolar are dissolved in organic solvent and components that are polar are mixed in with the aqueous solvent.

Because organic solvent is less dense it sits above the aqueous solvent. The exceptions are chloroform and dichloromethane which sits below aqueous layer.

Question 14

Back- extraction

Answer 14

When ionizable components of an extraction are dissolved in the aqueous solvent we can change the pH so that it reverts back to the nonionizable form putting it into the organic layer. This is a way to remove remove impurities.

Question 15

What is the process of distillation?

Answer 15

Liquid-liquid separation in which we separate two liquids using their differences in their boiling points.

Question 16

Describe the components and process of distillation.

Answer 16

The initial mixture is called the distilland. We boil it and the vapors move to the condenser ( jacketed tube) where a cooling agent moves through and condenses it back to a liquid where it collects in the receiving flask.

The process is measured by a thermometer located at the distillation adapter.

Within the distilland boiling chips or stir bars are added to allow even boiling and to prevent bumping ( formation of vigourous bubbles) or superheating.

Question 17

Describe the countercurrent flow of the cooling agent during distillation?

Answer 17

It runs from the lower condenser port to higher condenser port to allow maximal cooling of vapors.

Question 18

Purpose of a receiver adaptor in distillation

Answer 18

Can attach to the connection port and serves to direct the distillate to the receiving flask ( i.e. such as a vacuum).

Question 19

Simple distillation

Answer 19

The most basic type of distillation that happens between solvents with a greater than 25C difference in boiling point.

Occurs through simple boiling- condensation cycle.

Question 20

Vacuum distilliation

Answer 20

Used to separate liquids with a different in boiling points greater than 150C.

The pressure is reduced which allows the boiling point to be reduced.

Stir bars are used instead of boiling chips because the reduce pressure gets rid of the pockets within them.

Question 21

Fractional distillation

Answer 21

Distillation technique that uses glass or bead chips in the condenser to increase the surface area and increase condensation rate.

Uses multiple repetitive vaporization- condensation cycles which results more and more purified sample as it’s done.

Used for liquid-liquid samples with less than 25C difference in boiling points.

Question 22

Chromatography

Answer 22

Separation technique that utilizes one component of mixture interacting with mobile phase and the other acting with the stationary phase.

Question 23

What is the basis practice behind chromatography?

Answer 23

Resolution ( the separation of components of a mixture by preference of one component over the other). Quicker when there is a strong preference and when the stationary phase is longer.

Question 24

How is data portrayed in chromatography?

Answer 24

As a chromatogram with the signal intensity on the y-axis and the time on the x-axis.

The baseline doesn’t show any signal intensity and therefore represents no detection of sample.

Greater the peak, greater the sample detected has for the mobile phase.

Question 25

Are retention time and retention factor inverse or directly correlated with each other?

Answer 25

Inverse of each other.

Question 27

Describe the basis behind gas chromatography

Answer 27

An inert gas is the mobile phase and can be helium, argon, or nitrogen. The stationary phase is either a solid or liquid.

Separation is based on polarity and vapor pressure.

Question 28

GC-MS

Answer 28

Gas chromatography - mass spectroscopy , mass spectrometer is a type of a detector.

Question 28

Describe the process of using gas chromatography

Answer 28

1. Sample dissolved in a low B.P. solvent is injected into the inlet.
2. Sample is heated and vaporized
3. The carrier gas elutes it through the column.
4. The detector measures what elutes through to create a chromatogram.

• Components that are more volatile elutes faster while components that are less volatile elutes slower and gets stuck to or dissolved in the stationary phase.

Question 29

Normal phase v. Reverse phase chromatography

Answer 29

Normal phase chromatography is when the stationary phase is polar and the mobile phase is nonpolar.

Reverse phase chromatography is when stationary phase is nonpolar and mobile phase is polar.

Question 29

Analytical v. Preparative chromatography

Answer 29

Analytical chromatography is when we use the method to analyze the products of separation.

Preparative chromatography is used when we want to separate the components and then extract the desired component.

Question 30

Paper chromatography

Answer 30

Uses paper covered in a polar carbohydrate polymer with a nonpolar mobile phase.

The sample is spotted onto the paper chromatography and the mobile phase rises up via capillary action. The end of the mobile phase movement is the solvent front.

To detect the separation of the mixture we use a stain or visible color.

Question 31

Retention factor ( Rf)

Answer 31

Used to measure chromatography movement.

The ratio of the movement of the solute to the movement of the solvent front.

The range is from 0 to 1 with 0 being complete attraction to the stationary phase and 1 having complete attraction to the mobile phase.

Question 32

Thin- layer chromatography

Answer 32

The stationary phase is either a polar silica or alumina gel placed on an inert surface.

The process is the same as paper chromatography.

To see samples we can place under UV light and circle the spots that doesn’t show UV light. We can also use a chemical stain but this can irreversibly alter the functional groups.

Question 33

What’s the difference in TLC when we do analytical chromatography v. preparative chromatography

Answer 33

For analytical chemistry we use a thin TLC plate.

For preparative chemistry we use a thick TLC plate.

Question 34

How can we increase the amount of substance separated during column chromatography?

Answer 34

By increasing the length and cross- sectional area of the column.

Question 35

HPLC ( High Pressure Liquid Chromatography)

Answer 35

HPLC is similar to column chromatography except that the solvent mobile phase is pumped through at high pressure.

A common way to measure what elutes through is to subject it to UV light and measure how much UV light is absorbed which is seen as a peak on the chromatogram.

Question 36

What’s the difference in column chromatography when we do analytical chromatography v. preparative chromatography

Answer 36

For analytical chromatography we want to use smaller cross- sectional columns

For preparative chromatography we want to use larger cross- sectional columns.