MCAT Question of the Day

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Correct Answer: D. Ne and Kr

Two gases having the same temperature have the same average molecular kinetic energy. Therefore, (1/2)m1v1² = (1/2)m2v2², which gives m2/m1 = (v1/v2)². Since v1/v2 = 2, we have m2/m1 = 2² = 4. Among the available choices, the only gases with a mass ratio close to 4 to 1 are krypton (83.8 g/mol) and neon (20.2 g/mol).

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Correct Answer: B. E points to the right.

By convention, electric field vectors always point away from positive source charges and toward negative ones. Thus, in the region between the given point charges, the electric field points to the right: away from the positive and toward the negative.

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Correct Answer: A. [Ar]

A calcium atom normally contains 20 electrons, so if it acquires a charge of +2, it must have lost 2 electrons, leaving just 18. Only choice A has 18 electrons.

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Correct Answer: B. 10 N

What are the forces on the book? The weight, 40 N, and the normal force, N are downward forces, and the only upward force is our 50 N push. Since the bloc k is not accelerating vertically, the net vertical force must be zero. Therefore, 40 + N must be 50, which means N = 10 N.

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Correct Answer: A. 0 N

Since the velocity is constant, the acceleration a is zero, so using F = ma, we know that the net force must be zero as well.

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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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