MCAT Question of the Day

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. -5/4Q

The basic principle: the spheres are equal in size so when they touch (conduction of charge), the charge will be split equally between the 2 spheres (this is consistent with the Conservation of Charge). When the spheres are separated, each sphere will retain 1/2 of the charge (because they are identical in size).

1) X plus Y: When -Q touches +2Q, the overall charge of the 2 spheres would be -Q + 2Q = +Q but that charge is shared with 2 spheres so each sphere has a +1/2Q charge.

2) new X plus Z: Now +1/2 Q – 3Q = -5/2Q and half of this is -5/4Q

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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: D. A and B

In the situation described, there are 3 vectors and each of the 3 vectors are perpendicular to each other (you can imagine the relationship as being the x, y and z axes, or as governed by the right hand rule): the current (I), the electric field (F) and the magnetic field (B). The latter encircles the current carrying wire.

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Correct Answer: B. They all contain codons.

There are 3 types of RNA: (1) ribosomal RNA (rRNA); (2) messenger RNA (mRNA) and (3) transfer RNA (tRNA). RNA is a single-stranded polymer made up of four bases, namely uracil, adenine, guanine and cytosine. Only mRNA can contain a codon, a genetic code consisting of three base pairs that can be translated by the cellular machinery into an amino acid during protein synthesis. RNA elongates elongates from the 5? to 3? direction using a DNA strand with the opposite orientation as a template for transcription.

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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: A. Twice as much work is done on the smaller mass.

PHY 5.3

Simple way:

Use the Work-Energy Theorem where the variation of the kinetic energy of a system is equal to the work of the net force applied to the system. Thus the work is just the change in kinetic energy (i.e. final MINUS initial). But the final is 0 because the final velocity is 0 (at “rest”). So we get:

Work = -m(v²)/2

Looking at the equation, it is clear that doubling the velocity has a greater impact on the work as compared to doubling the mass.

Side Note: Friction always does negative work because it is opposite to the direction of motion. Similarly, work must be negative in this problem.

Alternate method:

Work = Force times distance (actually, displacement) = m (a) x

To determine the acceleration a (actually, the deceleration):

V² = Vo² + 2ax = 0 (each object has a final vel. of 0)

a = – (Vo²)/2x

Work = m [- (Vo²)/2x] x = -m(Vo²)/2

Now by testing the numbers given, you can see that the original velocity has a greater impact than the mass.

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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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