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Different in Retention

ACTSCI-BAJK9K

Column chromatography can be used to separate a mixture of different alcohols based on the relative polarity of their molecules. The column works by using a non-polar solid substrate called the stationary phase, and a polar liquid called the mobile phase.

How quickly an alcohol moves through the column will depend in large part on the relative polarity of the molecules. The more polar molecules will be attracted to the mobile phase, and will move quickly through the column.

The less polar molecules will be more strongly attracted to the non-polar stationary phase, and will move through the column more slowly. The time it takes a molecule to move through the system is called the retention time.

Table 1 shows the retention time for 5 different alcohols. The same mobile phase was used in all trials. Charts 1 - 4 show the results of running four different unknown mixtures through the column. Each of the mixtures is known to contain some combination of the known alcohols.

Which of the following statements BEST explains the difference in retention times between isopropanol and n-propanol?

A

Having the polar $-OH$ group in the middle of the isopropanol molecule makes the whole molecule more polar and increases the retention time.

B

n-propanol has a shorter retention time, due to the extra $-OH$ group that it has compared to isopropanol.

C

The longer carbon chain makes n-propanol less polar, and this reduces the attraction to the column and reduces the retention time of the molecule.

D

Having the highly polar $-OH$ group at the end of the molecule makes the whole molecule more polar. This reduces the retention time of the molecule.