Kaplan MCAT General Chemistry Review

Assuming that the same amount of cations and anions dissolve, we can refer to Ag⁺ and Br^- both as x.

K_sp = [x][x] = 7.7 × 10^-13

However, there is already 0.001 M Br^- in solution. To account for that, we’ll add it in to the K_sp equation.

K_sp = [x][x + 0.001 M] = 7.7 × 10^-13

We know 0.001 is much bigger than the square root of 10^-13. In other words, it’s much bigger than x, and x is insignificant in comparison. So we’ll approximate Br^-’s concentration as 0.001 M and solve for silver. The solubility of Ag⁺ is the same as AgBr’s because one molecule of Ag⁺ dissociates per molecule of AgBr.

We find that [Ag⁺] is 7.7 × 10^-10 M Ag⁺. This value can be converted to g/L by multiplying by the molar mass of silver, 107.9 grams/mole.

(7.7 × 10^-10 mol/L)(107.9 g/mol) = 8.3 × 10^-8 g/L

17. B

Formation of complex ions between silver ions and ammonia will cause more molecules of solid AgCl to dissociate. The equilibrium is driven toward dissociation, because the Ag⁺ ions are essentially being removed from solution when they complex with ammonia. This rationale is based upon Le Châtelier’s principle, stating that when a chemical equilibrium experiences a change in concentration, the system will shift to counteract that change. (A) is incorrect because the complex ions may interact with AgCl but this is not the major reason for the increased solubility. (C) and (D) are incorrect because the solubility of AgCl will increase, not decrease.

18. D

Detergents are compounds that contain a long hydrophobic chain with a polar functional group on one end. The long hydrophobic chains can surround grease and oil droplets, while the polar heads face outward and carry the particles in a solution of water. (A) describes micelles, which are very different in configuration. (B) is internally inconsistent because ionization creates two ionic particles. (C) is incorrect because although multiple detergent molecules form a spherelike shape with oil or grease droplets enclosed, the individual molecules themselves do not form ring structures.

Chapter 10: Acids and Bases

There are many ways that drugs can enter the human body. The route of administration of a drug is the path by which that drug comes into contact with the body. According to the U.S. Food and Drug Administration’s Data Standards Manual, there are no fewer than 110 distinct ways in which a drug can come in contact with and/or enter the human body in a local or systemic manner, including the catch-all route of “other.” Some drugs can be applied as drops, salves, or creams to mucus membranes. Others are injected. Some employ a transdermal patch, while others are eaten, drunk, or inhaled. You will be challenged in medical school to learn and recall the routes of administration for many commonly prescribed medications and treatments. This is one of the more daunting memorization tasks that will be demanded of you during medical school and residency.

The route of administration of a drug compound is related to both the location of its target tissue (local or systemic), as well as the chemical and physical properties of the compound. For example, compounds that are water-soluble can be administered intravenously (an aqueous solution dripped directly into the bloodstream), while those that are lipid-soluble can be administered transcutaneously (from, say, a patch or a cream) or orally (in a pill or liquid suspension). The polarity, size, and charge of the drug compound will determine its solubility in polar or nonpolar environments and will be major contributing factors to the most effective and efficient route of administration.

Whether a drug compound has an ionic charge is usually a function of the acidic or basic nature of the compound. For example, a basic organic compound that is water-insoluble when neutral can be reacted with an acid to form a salt that, because it is ionic, will be water-soluble. Correspondingly, an acidic organic compound that is water-insoluble when neutral can be reacted with a base to form a water-soluble salt. On the other hand, the protonated (acidic) form of an organic compound can be reacted with a base to neutralize the compound and release it from its salt, changing (and usually reversing) its solubility in water.