Kaplan MCAT General Chemistry Review

Sodium fluoride is used in toothpastes to reduce the virulence of bacteria that cause dental caries, also known as cavities. Most U.S. residents are exposed to sodium fluoride, and its use has been correlated with a decline in the incidence of dental caries in most of the population. Although there is debate over the mechanism by which sodium fluoride acts to reduce dental caries, it has been established that the fluoride ion is the main contributor to its efficacy.

Fluoride is the ionic form of the element fluorine. The fluoride ion has a high degree of electronegativity and so holds a negative charge in solution. It thus forms relatively stable bonds with positive ions such as H⁺ and Na⁺. Fluoride inhibits carinogenic bacteria from metabolizing carbohydrates and thus prevents subsequent production of acid in the oral cavity. The decrease of acidity reduces erosion of tooth enamel, which would otherwise lead to fissures and irregular surface changes in the tooth.

In a variety of laboratory studies, certain types of Streptococci bacteria, a main culprit in the formation of caries, are adversely affected when exposed to fluoride ion concentrations of varying levels. In particular, it was found that Streptococcus mutans, the more virulent species of the Streptococci, produces less acid when exposed to fluoride than Streptococcus sobrinus, the less virulent form. The reasons for the link of reduced acid production to fluoride levels are still unclear.

46. Based on the passage, which of the following can be definitively stated about the action of fluoride ions on oral health?

A. F^- prevents bacteria from forming dental caries.

B. F^- kills populations of bacteria that cause dental caries.

C. F^- is related to killing populations of bacteria that form dental caries.

D. F^- is related to less acid production and reduces the risk of dental caries.

47. Based on the passage, what is a possible mechanism by which fluoride could act upon Streptococci bacteria to reduce their production of acid?

A. Fluoride adds enamel to the developing tooth structure.

B. Fluoride fills and closes fissures within the enamel topography.

C. Fluoride adds electrons to bacterial respiration reactions.

D. Fluoride pulls electrons from bacterial respiration reactions.

48. What property contributes to the high electronegativity found in the fluorine atom?

A. Small atomic radius

B. Small number of protons in the nucleus

C. Large number of electrons in the orbit

D. Large number of electron shells in the orbit

49. Which of the following is the correct electronic structure notation for fluoride?

A. 1s²2s²2p⁵

B. 1s²2s²2p⁴

C. [He]2s²2p⁶

D. [Ne]2p⁶

50. According to Heisenberg, what can be accurately, quantitatively determined in a neutral atom when the location of the electron is found?

A. Electron momentum

B. Velocity of electron

C. Mass of electron

D. None of the above

51. What is the effective nuclear charge on the outermost electron in fluoride?

A. 9

B. 2

C. 7

D. 8

52. What electrons are most available for bonding in the fluoride ion?

A. 3s

B. 2p

C. 1s

D. 2s

Answers and Explanations

PRACTICE SECTION 1

ANSWER KEY

1. A

2. C

3. D

4. A

5. B

6. D

7. C

8. C

9. B

10. B

11. D

12. B

13. D

14. A

15. A

16. B

17. C

18. B

19. C

20. C

21. A

22. C

23. D

24. A

25. C

26. D

27. B

28. A

29. B

30. A

31. B

32. C

33. D

34. C

35. A

36. C

37. B

38. A

39. C

40. D

41. C

42. B

43. A

44. A

45. C

46. B

47. B

48. C

49. B

50. A

51. D

52. C

PASSAGE I

1. A

There are two concepts involved here. The first is that both of these species are strong acids, so they will fully ionize in solution; the bivalent species has two H⁺ ions for every one H⁺ ion of the monovalent species, so it will produce a more acidic solution. Second is the idea that a more acidic solution will have a lower pK_a.

2. C

It is given in the passage that the pH of normal rain is 5.2, so this problem is simply asking you to convert pH = 5.2 into [H⁺]. Using the Kaplan logarithm estimation procedure on the answers, it is clear that the only concentration that will produce a pH of 5.2 is (C).

3. D

It is important to know that every Arrhenius acid is also a Brønsted-Lowry acid, and that every Brønsted-Lowry acid is also a Lewis acid (the same idea applies for bases). This means that (B) and (C) can be eliminated. Sulfuric acid produces protons, so it is qualified as an Arrhenius acid, and we then know it can be called a Brønsted-Lowry or Lewis acid as well. All of the items are correct.

4. A