Kaplan MCAT General Chemistry Review

(C) correctly describes the underlying forces of hydrogen bonding. Electronegative atoms bound to hydrogen disproportionately pull covalently bonded electrons toward themselves, which leaves hydrogen with a partial positive character. That partial positive charge is attracted to nearby negative or partial-negative charges, such as those on other electronegative atoms. (A) is not true; hydrogen has little electronegativity and does not hold its valence electrons closely. (B) is a true statement but not descriptive enough to explain hydrogen bonding. (D) is not correct; although these bonds are highly polarized, they are not ionic.

9. D

First recall that ammonium is NH₄⁺, while ammonia (commonly confused) is NH₃. It helps to associate the suffix -ium with a charged form of the molecule. Once you remember that ammonium is NH₄⁺, eliminate any answer choice that only accounts for three bonds [answer choice (A)]v. Next, it helps to recall that ammonium is formed by the association of NH₃ (uncharged, with a lone pair on the nitrogen) with a positively charge hydrogen cation (no lone pairs). In other words, NH₃ is a Lewis base, while H⁺ is a Lewis acid. This type of bonding, between Lewis acid and base, is a coordinate covalent bond. Thus, you know that there is one coordinate covalent bond in this molecule, making (D) the correct answer.

10. C

This question addresses the issue of when the octet rule can be violated. All atoms that are in the third or higher period have d-orbitals, each of which can hold 10 electrons. The typical eight “octet” electrons reside in s- and p-orbitals. (A) and (B) do not explain why more than eight electrons can be held. Electronegativity is irrelevant to whether or not an atom can exceed the octet rule, making (D) incorrect.

11. C

All of the listed types of forces dictate interactions among different types of molecules. However, noble gases are entirely uncharged and do not have polar covalent bonds, ionic bonds, or dipole moments. Recognize that the only types of forces listed that could relate to noble gases could be van der Waals forces (A) or dispersion forces (C). Of these two, van der Waals forces is an umbrella term that includes both dispersion and dipole–dipole interactions (which you’ve already eliminated). Because not all van der Waals forces apply, rule that out as the correct answer and stick with dispersion forces. Dispersion forces are a specific type of interaction that occurs among all bonded atoms due to the unequal sharing of electrons at any given moment in the electron’s orbit. This unequal sharing allows for instantaneous partial positive and partial negative charges within the molecule. Though these interactions are small, they are necessary for liquefaction.

12. B

The key to this question is understanding the pattern of filling for s- and d-orbitals among the transition metals. There are 24 electrons in chromium, 6 more than are present in argon. Where will these six electrons lie? It is advantageous for d-orbitals to be half filled. Therefore, one electron will fill each of the five 3d-orbitals. It will take less energy to put the sixth electron into an s-orbital than it would to add it to one of the d-orbitals that already has an electron (and force it to be no longer half filled).

13. A

In this Lewis diagram, the PO₄^3- molecule has an overall formal charge of -3. The four oxygens each would be assigned a formal change of -1, based on the following formula: Formal charge = V (valence electrons in the free atom) -½ N_bonding (electrons shared in bonds) -N_nonbonding (lone pairs/free electrons). For each oxygen, we calculate: FC = 6 - ½ (2) - 6 = -1. For the central phosphorus, assume then that with a total formal charge of -3 and four oxygens with a change of -1 each, the phosphorus must have a formal charge of +1. Alternatively, calculate its formal charge as FC = 5 - ½ (8) - 0 = +1. Considering this molecule’s other resonance structures, you’d come to the same conclusion—that phosphorus is the most positive atom.

14. C

The reaction in this question shows a water molecule, which has two lone pairs of electrons on the central oxygen, combining with a free hydrogen ion. The resulting molecule, H₃O⁺, has formed a new bond between H⁺ and H₂O. This bond is created through the sharing one of oxygen’s lone pairs with the free H⁺ ion. This is essentially a donation of a shared pair of electrons from a Lewis base (H₂O) to a Lewis acid (H⁺, electron acceptor). The charge in the resulting molecule is +1, and it is mostly present on the central oxygen, which now only has one lone pair. This type of bond, formed from a Lewis acid and Lewis base, is called a coordinate covalent bond.

15. B