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Moderate

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.

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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.