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Solvents and Solubility

APCHEM-9JBDBH

Nonpolar solvents such as hexane (${C}_{6}{H}_{14}$) generally are immiscible with water; thus, when water and hexane are placed together in the same container, they will separate into two layers as shown below. Hexane (density = 0.69 g/mL) will form the top layer.

Some substances are soluble in both solvents to differing extents. Consider the following organic acids, all of which exhibit some ability to dissolve in both water and in hexane.

• Ethanoic acid: ${CH}_{3}COOH$
• Propanoic acid: ${CH}_{3}{CH}_{2}COOH$
• Butanoic acid: ${CH}_{3}{CH}_{2}{CH}_{2}{COOH}$
• Pentanoic acid: ${CH}_{3}({CH}_{2})_{3}COOH$

A student designs an experiment to determine the relative solubilities of the four acids in hexane and in water. She wants to express her results in the form of a ratio, represented by $k$, defined as follows.

$$k=\cfrac { \text{molar concentration of acid in hexane} }{\text{ molar concentration of acid in water} }$$

Which of the following is expected to have the greatest value of $k$, for the reason given?

A

All will behave alike. All four compounds contain a highly polar carboxylic acid group, which is attracted to water, and all have a nonpolar "tail" that is attracted to hexane.

B

Pentanoic acid, ${CH}_{3}({CH}_{2})_{3}COOH$, because it is the weakest acid of the four.

C

Pentanoic acid, ${CH}_{3}({CH}_{2})_{3}COOH$, because it has the largest nonpolar portion.

D

Ethanoic acid, ${CH}_{3}COOH$, because it is the strongest acid.