MCAT Question of the Day

Correct Answer: B. 7

There is one s orbital, and there are three p orbitals, five d orbitals, and seven f orbitals. Thus, the answer is B. Do not confuse the number of orbitals in a subshell with the number of electrons the subshell can hold. Each orbital can hold two electrons, so the capacity of an nf subshell is 14 electrons.

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Correct Answer: B. 1 to 2

Alpha decay results in the loss of two protons, while ordinary beta decay results in the gain of one proton. Since isotopes of the same element have the same number of protons, the number of protons lost by alpha decay must equal the number gained by beta decay. Therefore, twice as many beta decays as alpha decays must occur, so the ratio of alpha to beta decays is 1:2.

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Correct Answer: B. The enthalpy and entropy changes are negative

If the products have more stable bonds, that means they have less energy, and that energy must have been released, hence the change of enthalpy is negative. If the products are in a more orderly arrangement than the reactants, then the entropy decreased; thus the change in entropy is also negative, and the only answer left is B.

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Correct Answer: A. Excited electrons dropping to lower energy levels.

The line spectra of atoms are the result of the photons emitted when excited electrons drop to lower energy levels.

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Correct Answer: C. 8 grams.

From the balanced equation, we can see that 4 moles of Ag are produced for every 1 mole of O2. Therefore, if 1 mole of Ag is produced, then (1/4) mole of O2 is produced. The mass of (1/4) mole of O2 is (1/4)(2)(16 g/mol) = 8 grams.

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Correct Answer: D. [Ar] 3d9 4p1

The ground state for Cu is [Ar] 4s1 3d10. Transition metals always lose their valence s electrons before any d electrons, so the ground-state electron configuration for Cu⁺ [(I) means +1] is [Ar] 3d10. The only excited configuration for Cu⁺ must be [Ar] 3d9 4p1. An alternate strategy is to look at the number of electrons, as Cu⁺ has 28, and this helps eliminate A and C as they do not have 28 electrons.

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Correct Answer: B. The energy of the activated complex will decrease, and the reaction rate will increase.

A catalyst lowers the activation energy of a reaction, causing an increase in the reaction rate.

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Correct Answer: B. KE = (1/2)mv²

The rate of expansion (or diffusion) is directly proportional to the mean velocity of the gas molecules. The only equation that has a term for velocity is the one in choice B.

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A.

Zero order reactions proceed at a constant rate. The time it takes to go from one-half to one-quarter of the original amount will be only half the time needed to go from all of the reactants to one-half of them. (Fun Fact: A first-order reaction would take 5 minutes.)

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Correct Answer: D.

In pyrrole, the sigma bonds around nitrogen are formed by sp2 hybrid orbitals, not sp3. A nitrogen electron participates in each of the sigma bonds. The remaining nitrogen electrons are in a nitrogen 2p orbital which overlaps with the carbon 2p orbitals in the ring to form pi bonds and the cyclic compound, delocalizing the lone pair. In pyridine, the nitrogen atom donates one electron to each of the two sigma bonds by use of sp2 orbitals. The third sp2 orbital contains two electrons, which are for acids. One electron of nitrogen is present in a p orbital, which interacts with the carbon p orbitals to form the pi bond.

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