Kaplan MCAT General Chemistry Review

If you ignore the digression about astronomy, the question is straightforward and does not require referring back to the passage. It simply asks what color of visible light corresponds to 656.3 nm. The visible light spectrum covers 400-700 nm. Using the ROYGBIV mnemonic and prior knowledge that infrared wavelengths are longer than visible wavelengths, you know that red is on the 700 nm end, and that violet is on the 400 nm end.

14. B

This image is an absorption spectrum, in contrast to the emission spectrum presented in the passage. Think of it as the inverse of the emission spectrum. We cannot tell from the passage what a Lyman or Bohr series looks like.

15. C

Deuterium is “heavy” hydrogen, with one neutron, one proton, and one electron. The Balmer series concerns only electron transitions, so there will be no change in the number of peaks. In other words, there will be the same number of peaks, but with more splitting; the nucleus is heavier, which will slow down the transitions. This effect is visible at high resolutions.

16. A

A full explanation is out of the scope of the MCAT Physical Sciences section, but you should know from general physics that Bohr’s model was incomplete because it didn’t incorporate the theory of relativity. We know from the question stem and equation that the Balmer series works only for wavelengths related to the quantized energy levels (B). Though it is not stated explicitly in the passage, Bohr’s quantitative model incorporates atomic number (C) (more detail is unnecessary for now; in short, it affects the atomic radius). Hydrogen atoms have only one electron (D), and we have no indication that other particles are involved.

17. C

According to Bohr’s model, the energy differences between quantized energy levels become progressively smaller the further away the electron moves from the nucleus. Textbooks will sometimes describe this concept as if the energy levels themselves are “narrowing.” (If energy levels are depicted as rings around a nucleus, they will appear to be closer together as you move farther from the nucleus.) Read carefully. (B) describes a drawing like this one, but the graphical representation of this concept does not really explain the difference in energy. We do not have enough information in the passage to determine whether the energy levels themselves are a greater or smaller distance apart. (A) is the opposite of (B), intended to stump readers who might not read through to (C) and (D). Because (C) refers directly to energy differences between energy levels, it is more accurate. (Remember E = hf = h × [speed of light/ wavelength].) (D) is its opposite.

QUESTIONS 18–22

18. D

Only (D) gives a response where both kinds of particles have a mass. Neither neutrons nor gamma rays have mass and so they are unchanged by the actions of a particle accelerator. The dependence on mass arises because a particle accelerator works by means of high-energy ideally elastic collisions. Also, if there is no mass, both kinetic energy and momentum are undefined.

19. B

A solution is available here which doesn’t require an equation. Both the acid and base are monoprotic, thus there are no multiple dissociations. Notice that the acid is twice as concentrated as the base. Thus, double the amount of base will be needed to neutralize the acid or 2 × 15 mL = 30 mL. If you forgot this factor of 2, you might have chosen (A), and if you squared the 2 out of uncertainty (only in physics do you square things when in doubt) you might have chosen (D). C is the total volume of the solution, not the volume just of the base.

20. A

The ideal gas theory assumes that the particles have large interatomic distances and a relative absence of intermolecular forces. This is more true at lower pressures than at higher ones, thus eliminating (C) and (D). (B) implies that high pressure squishes the molecules. According to the kinetic theory of gases, individual atoms are treated as incompressible point properties. The main effect of the pressure is to reduce the interatomic distance, not the intra-atomic distance (i.e., the atomic radius).

21. A

Molecules can only be nonpolar if they have total symmetry around the central atom. Bent molecules lack this symmetry (think of a molecule of water with two lone pairs of electrons and two hydrogen atoms extending from the central oxygen atom) and so a dipole is created which causes polarity in the molecule. A diatomic covalent (B) molecule (such as H₂ or O₂) is perfectly symmetrical. (C) and (D) are incorrect because, although dipoles can exist in these configurations, symmetrical arrangements are also possible.

22. C