Exp 2 Chromatography Pre Lab Problems PDF

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Pre lab worksheet with answers for CHEM131L...

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Name: _______________________________________

Lab Section: ___________

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Experiment 2: Paper Chromatography of Food Dyes Pre-Lab Part 1: Principles of Chromatography Making sure the experiment is set up properly is important because an improper setup can affect the results. Let’s review a couple of key things to watch out for with paper chromatography. 1. When placing the chromatography paper with sample on it into the developing chamber with the mobile phase solvent, why is it important to ensure the solvent is below the origin line?

2. Why should you use pencil and not pen when labeling the chromatography paper?

3. After removing your stationary phase paper out of your beaker why is it important to immediately draw the solvent from line on the paper?

1

Part 2: Molecular Polarity and Intermolecular Interactions Chromatography works on the principle of polarity. How polar and nonpolar compounds interact with the stationary and mobile phases influences how far compounds travel up the paper. In order to help select a solvent that will separate your ionic dyes you should understand the polarity of your solvents. Below are the chemical structures for the solvents you are going to be using in this experiment, the charges have been included to help you better understand the polarity of the molecules. δ− O

δ− H

δ+

H

δ+

O

O H

δ−

δ−

H3C

δ+ water

C

Na Cl

Hδ+

δ+

O

δ− sodium chloride

acetic acid

N H

δ+

δ+

H H

δ+ ammonia

δ+

CH

H3C

CH3

isopropyl alcohol

4. There are four solvents to test: aqueous 0.01 (w/w) % NaCl, aqueous 0.05 M acetic acid, aqueous 0.1 M ammonia, and isopropyl alcohol. What does the term aqueous mean?

5. Consider the structures of the solvents above. Classify the molecules as nonpolar covalent, polar covalent, or ionic. Briefly describe how you came up with your choices. Nonpolar

Polar

Ionic

6. Consider the structures of the solvents above. Circle the compounds that are capable of Hbonding? Briefly describe how you came up with your choices. Water

Sodium Chloride

Acetic Acid

2

Ammonia

Isopropyl Alcohol

Part 3: Polarity and its role in chromatography Molecules with similar polarity are better able to interact with each other. So the more polar compounds travel more slowly with the solvent than the less polar solutes. Less polar compounds do not interact as much so they are able to travel further up the paper. 7. Ink from different pens is being separated via paper chromatography. Each dye from the ink has been designated by a letter A-C. Rank the dyes by most polar to least polar. Most

Least

A

B

C

8. In your experiment you are using ionic dyes, would you expect the dye that travels furthest up the plate to have more or less H-bonding and ionic character than a dye that does not travel as far? Explain.

9. You can control how far a dye travel up the stationary phase by alternating the mobile phase. You want to find a mobile phase that gives good separation between compounds. Based on the poor separation should add a more polar or nonpolar solvent to increase separation?

A

3

B

C

Part 4: Retention Factor The retention factor is denoted by Rf the distance traveled by the compound ฀฀฀ ฀ = the distance traveled by the solvent 10. When calculating the Rf, why does the 0cm start are the origin line and not the bottom of the paper?

11. Calculate the Rf values for each of the dyes.

Compound

A B C

A

B

C

4

Distance traveled by spot

Distance traveled by solvent

Rf...