Kaplan MCAT General Chemistry Review

Periods 2 and 3 of the periodic table, which contain only elements in the s-block and the p-block, are the only periods that actually contain exactly eight elements. If most of the elements in the d-block and the f-block were discovered, it would become obvious that elements do not occur in matching octaves. (A) is incorrect because the discovery of all of the s-block and p-block elements would create more matching octaves (although the theory would have to be modified to accommodate the noble gases). (C) is incorrect because the law of octaves does not exclude the possibility of electrons, as long as the electrons are not organized as in Bohr’s theory. (D) is incorrect for the same reason as (C).

32. C

Mendeleev’s table was organized in terms of atomic mass rather than atomic number. Item I did require modification because it was subsequently discovered that an element is characterized by its number of protons (rather than its mass). Item II did not require direct modification of the table and the existing entries were left intact; the only change was the knowledge that future entries would be organized according to their outer orbitals. Item III only strengthened Mendeleev’s original model, since the discovery of gallium fulfilled his prediction that two elements exist between zinc and arsenic. Items II and III did not require modification, so (C) is the answer.

33. D

The passage states that Newlands’s table contains all of the elements that had been discovered at the time. Because his table contains no noble gases, we can assume that they had not been discovered. The table contains uranium, which is an f-block element (A), and several instances of halogens and metalloids [(B) and (C)] are evident throughout the table.

34. C

An element’s first ionization energy, which is defined as the tendency of the element to donate an electron, decreases with increasing atomic number within a period. Because Mendeleev’s predicted element had a higher atomic number than calcium, it is less likely to lose an electron. Atomic radius and ionic radius [(A) and (B)] both decrease with increasing atomic number. Electron affinity increases with increasing atomic number within the same period (D).

35. A

Atomic radius increases as an element gains energy levels and decreases as an element gains protons and electrons; you should know that the largest elements are those that have more energy levels and are found near the bottom of the periodic table. Uranium is the only element on Newlands’ table that has seven energy levels, meaning it has the largest atomic radius.

36. C

If Mendeleev had created his table in response to Newlands’ theory, then Mendeleev would clearly never have made his breakthrough without Newlands’ contribution; if this were the case, it’s reasonable to say that Mendeleev simply modified the law of octaves, which was eventually refined to produce the modern version. (A) is incorrect because it does not suggest that Newlands had any impact on the development of today’s periodic table. (B) also makes it unlikely for Newlands to have made a direct contribution to the evolution of the system, because Mendeleev only learned about Newlands’ work after he had already invented his own table. (D) clarifies the fact that Mendeleev was the first person to publish the modern periodic table and does not suggest that Newlands made any contribution to these findings.

PASSAGE V

37. B

This is a simple ideal gas law question. Using the equation PV = nRT, we are able to substitute: (1 atm)(V) = (1 mole) (R)(318 K), which simplifies to 318R. The answer does not need to be simplified past 318R (you are not allowed to use a calculator during the exam and multiplication by R takes too much time by hand).

38. A

This question tests your knowledge of the ideal gas law. The law shows that volume and temperature have a direct linear relationship (PV = nRT), meaning that as volume increases, so must T (assuming isobaric conditions). (B) shows an indirect linear relationship, (C) an exponential one, and (D) a logarithmic one.

39. C

In Experiment 2, we see that V₁ has been halved. So using the knowledge that temperature has been held constant, we know that the pressure must be double what it originally was to maintain the ideal gas law. So the right answer is 2 atm. The other choices all would violate the ideal gas law.

40. D

The a in the van der Waals equation accounts for the attractive forces between gas molecules. The b in the equation accounts for the actual volume that the molecules occupy (B). It isn’t necessary to memorize the van der Waals equation for Test Day, but do know what a and b correct for in the equation.

41. C

The key here is realizing that 64 g O₂ is 2 mole O₂. Substituting the values given into this law, we get: (2 atm)(3 L) = (2 mole)(R)(T ). This simplifies to 3/R K.

42. B