Читаем Kaplan MCAT General Chemistry Review полностью

Kaplan MCAT General Chemistry Review

Ammonia is very similar in structure to water. Because of this similarity, many biologists have wondered whether ammonia would be a suitable substitute for water in living systems. The ammonia molecule is composed of hydrogen atoms covalently bonded to nitrogen. As the oxygen in the water molecule has a slightly negative charge, so does the nitrogen atom in ammonia. Some scientists have argued that ammonia-based life could evolve on other planets in a similar manner as life developed on Earth. Others argue that ammonia’s heat of vaporization, 295 cal/g, is low compared with water, making it an unlikely candidate for the evolution of life. Modern science agrees that ammonia-based life on other planets will probably not be found to have evolved, if it exists, in the same manner as life on Earth.

31. One could infer from the passage that no form of life based on ammonia has yet been found because its

A. evaporation rate would be too high.

B. condensation rate would be too high.

C. rate of deposition would be too high.

D. rate of sublimation would be too low.

32. Had water not formed a crystalline lattice upon freezing and instead followed the common phase-change pathways of most other compounds, one could logically infer that

A. life could not have evolved in a liquid environment.

B. life would have evolved in a gaseous environment.

C. soils would hold greater amounts of liquid water.

D. soils would hold greater amounts of gaseous water.

33. When the water molecules shown in the previous illustration undergo sublimation, what BEST explains this phenomenon?

A. The attractive forces between the water molecules overcome the kinetic energy that keeps them apart.

B. The kinetic energy of the water molecules overcomes the attractive forces that keep them together.

C. The hydrogen bonds between the water molecules form at a more rapid rate in the solid phase.

D. The hydrogen bonds between the water molecules form at a more rapid rate in the liquid phase.

34. A change in which intrinsic property of water would MOST affect its polarity?

A. Atomic electronegativity

B. Chirality

C. Intermolecular forces

D. Solubility

35. How would one explain the negative slope of the water-solid equilibrium line in the phase diagram for water, shown below?

A. Water is more dense at 4° Celsius than at 0.

B. Water’s solid lattice collapses under pressure.

C. The triple point determines the slope of the phase-change line.

D. Liquid water is less dense than ice.

36. According to the passage, which of the following is TRUE about a molecule of water undergoing evaporation?

A. The water molecule has more energy than an ammonia molecule.

B. An average water molecule is moving faster than an average molecule of ammonia.

C. The water molecule has less energy than an ammonia molecule.

D. Both A and B

37. What would the addition of an ionic compound do to the lattice structure shown in the illustration above?

A. Collapse the structure

B. Enhance cohesive forces in the structure

C. Enhance crystallization

D. Both B and C

PASSAGE V (QUESTIONS 38–45)

There are many elements in the periodic table that play an integral role in the everyday functions of the human body. While the most obvious of these are carbon, hydrogen, nitrogen, and oxygen, we also have various essential uses for phosphorus. Its physical and chemical properties have made it the perfect candidate to play a role in the molecule that acts as the body’s primary location for short-term storage of energy, adenosine-5?-triphosphate (ATP).

Some metabolic syndromes can cause a phosphate ion to be drawn from the blood into the bones and teeth, where the majority of phosphorus exists. The ensuing deficiency of phosphate in the blood can lead to dysfunction of the brain and muscle tissue, which can cause death in severe cases. Scientists are attempting to cure these syndrome by designing a biologic molecule whose properties and actions are similar to those of phosphate. One way to do this is to find an element that has similar physical and chemical properties to phosphorus.

38. Which of the following is MOST likely to act as a stronger oxidizing agent than phosphorus?

A. Na

B. Cs

C. O

D. Bi

39. Which of the following, on average, has more space between two nuclei placed side-by-side than phosphorus?

I. K

II. Pb

III. F

A. I only

B. I and II

C. II and III