MCAT Question of the Day

Correct Answer: B. As pKa increases, acidity decreases.

A very low acid dissociation constant Ka means that the acid is weak and thus very little product is made. Since pKa is -logKa, the negative sign means they are always opposites. Thus low Ka means high pKa.

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Correct Answer: B. B

SN2 reactions proceed with inversion of configuration through backside displacement, so molecule A cannot be formed in this case. Molecule C is the product from elimination.

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Correct Answer: B. sp² and trigonal planar

On the surface: A carbocation is bonded to 3 substituents (i.e. ORG 4.2.1) and so it is in the middle of a flat triangle: trigonal planar.

When carbon has:
4 subtituents: sp³ and tetrahedral (109.5º)
3 subtituents: sp² and trigonal planar (120º)
2 subtituents: sp and linear (180º)

Going Deeper: There are three bonding electron pairs around the carbon atom, and these can be placed as far apart as possible in an sp² trigonal planar framework. The empty orbital, which results in the charge, is the 2pz atomic (unhybridized) orbital (ORG 1.2, 1.3; CHM 3.5).

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Correct Answer: A. A

Due to the angle strain, deviating most from the ideal angle of 109.5°, as found in cyclohexane C, cyclobutane A (ring strain, very unstable) has the highest heat of combustion per CH2 (the most energy released).

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Correct Answer: C. separating liquids based on their different boiling points.

Using fractional distillation one can increase the temperature of a liquid mixture and as the different liquids vaporize at their respective boiling point temperatures one can collect the vapor by condensing it and collecting the desired liquid at the corresponding temperature.

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Correct Answer: B. Beta particle

Since the radioactive emission angled up toward the positive plate, it must be oppositely charged, that is, negatively charged. The only negatively charged species in the answer choices is the beta particle which is, by definition, an electron or a positron. Note that electromagnetic radiation like gamma rays (particles) and infrared radiation are not deflected in electromagnetic fields.

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Correct Answer: D. Decreased temperature

Since the reaction is exothermic (negative enthalpy), heat can be considered a product in this reaction. A decrease in temperature will result in the equilibrium shifting to the right to bring heat to the system resulting in an increase in NO production, in accordance with Le Chatelier’s principle. A change in overall pressure of the system will not affect the equilibrium because there is an equal amount of moles of gas on both sides of the equation providing an even pressure. A catalyst affects the rate of a reaction not thermodynamic properties such as equilibrium.

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Correct Answer: C. a net negative charge.

A pH above the isoelectric point means that the amino acid is in relatively basic conditions. A base will accept a proton (H⁺) from the amino acid, leaving the amino acid negatively charged.

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Correct Answer: C. The other atom involved in the hydrogen bond (not the hydrogen atom) must be covalently bonded to a hydrogen atom.

Answer choice C. is not necessary for the formation of a hydrogen bond. A classic example is that of ethers. Ethers are water soluble because the oxygen in ether, which is covalently bonded to two carbons, can hydrogen bond with the hydrogen in H2O. Note regarding answer choice C.: the 3-D shapes of many proteins and DNA is due in part to intramolecular H-bonds.

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Correct Answer: D. ²²⁶88Ra

Equation I: ²³⁸92U -> ^xyZ + 3⁴2He + 2⁰-1e^– + 3 (gamma rays)

Since the sum of the atomic numbers and mass numbers on either side of the equation must be equal:

238 = x + (3 x 4) + (2 x 0) + (3 x 0)
x = 226
92 = y + (3 x 2) + (2 x -1) + (3 x 0)
y = 88

Thus, from the answer choices, Z = Ra; note that gamma rays are a form of electromagnetic radiation and thus have no charge and no mass.

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