MCAT Question of the Day

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. None of the above.

Only temperature will change the Ka, Kb, Kw, Keq, and Ksp. The Ksp will be constant and not change under the conditions provided in the answer choices.

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Correct Answer: C. I and III only.

According to ideal gas assumptions, all gas molecules are very far away from other gas molecules and their volumes are negligible. Should two gas molecules collide, their collision will be elastic, meaning no kinetic energy is lost (the average kinetic energy can only be changed by heating/cooling the gas). While it is true pressure is caused by the collision of gas molecules against the walls of the container, this is not an assumption for an ideal gas, but rather a general fact of nature. We typically want our ideal gases at low pressures too.

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