Ch.13 - Properties of Solutions

Problem 91a

Chapter 13, Problem 91a

The 'free-base' form of cocaine (C17H21NO4) and its protonated hydrochloride form (C17H22ClNO4) are shown below; the free-base form can be converted to the hydrochloride form with one equivalent of HCl. For clarity, not all the carbon and hydrogen atoms are shown; each vertex represents a carbon atom with the appropriate number of hydrogen atoms so that each carbon makes four bonds to other atoms
(a) One of these forms of cocaine is relatively water-soluble: which form, the free base or the hydrochloride?

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Identify the two forms of cocaine: the free-base form (C_{17}H_{21}NO_{4}) and the hydrochloride form (C_{17}H_{22}ClNO_{4}).

Understand that solubility in water is influenced by the polarity of the molecule. Polar molecules are generally more soluble in water.

Recognize that the hydrochloride form of cocaine is a salt, which typically increases its polarity and, consequently, its solubility in water.

Consider the role of the hydrochloride group (Cl^-) in the hydrochloride form, which can form ionic interactions with water, enhancing solubility.

Conclude that the hydrochloride form of cocaine is more water-soluble than the free-base form due to its ionic nature and increased polarity.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Solubility and Polarity

Solubility refers to the ability of a substance to dissolve in a solvent, which is often influenced by the polarity of the molecules involved. Polar substances tend to dissolve well in polar solvents (like water), while nonpolar substances are more soluble in nonpolar solvents. The presence of functional groups, such as -Cl in hydrochloride, can increase polarity and thus enhance water solubility.

Acid-Base Chemistry

Acid-base chemistry involves the transfer of protons (H+) between substances. In this context, the free-base form of cocaine can accept a proton from HCl to form the hydrochloride salt. This protonation alters the molecule's properties, including its solubility, as the resulting salt is typically more soluble in water due to increased ionic character.

Ionic vs. Molecular Compounds

Ionic compounds, such as hydrochloride salts, are formed from the electrostatic attraction between positively and negatively charged ions. These compounds generally have higher solubility in water compared to molecular compounds, like the free base form, which are held together by covalent bonds. The ionic nature of hydrochloride enhances its interaction with water molecules, leading to greater solubility.

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An “emulsifying agent” is a compound that helps stabilize a hydrophobic colloid in a hydrophilic solvent (or a hydrophilic colloid in a hydrophobic solvent). Which of the following choices is the best emulsifying agent? (a) CH3COOH, (b) CH3CH2CH2COOH, (c) CH3(CH2)11COOH, (d) CH3(CH2)11COONa.

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Proteins can be precipitated out of aqueous solution by the addition of an electrolyte; this process is called “salting out” the protein. A friend of yours who is taking a biochemistry class says that salting out works because the waters of hydration that surround the protein prefer to surround the electrolyte as the electrolyte is added; therefore, the protein’s hydration shell is stripped away, leading to protein precipitation. Another friend of yours in the same biochemistry class says that salting out works because the incoming ions adsorb tightly to the protein, making ion pairs on the protein surface, which end up giving the protein a zero net charge in water, and therefore leading to precipitation. Discuss these two hypotheses. What kind of measurements would you need to make to distinguish between these two hypotheses?

Textbook Question

The 'free-base' form of cocaine (C₁₇H₂₁NO₄) and its protonated hydrochloride form (C₁₇H₂₁NO₄) are shown below; the free-base form can be converted to the hydrochloride form with one equivalent of HCl. For clarity, not all the carbon and hydrogen atoms are shown; each vertex represents a carbon atom with the appropriate number of hydrogen atoms so that each carbon makes four bonds to other atoms (e) How many mL of a concentrated 18.0 M HCl aqueous solution would it take to convert 1.00 kilograms (a 'kilo') of the free-base form of cocaine into its hydrochloride form?

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A supersaturated solution of sucrose (C₁₂H₂₂O₁₁) is made by dissolving sucrose in hot water and slowly letting the solution cool to room temperature. After a long time, the excess sucrose crystallizes out of the solution. Indicate whether each of the following statements is true or false: (b) After the excess sucrose has crystallized out, the system is now unstable and is not in equilibrium.

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