Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 61:

  

  The two molecules above can best be described as:

  A. Structural isomers

  B. Diastereomers

  C. Enantiomers

  D. Conformational isomers

  Correct Answer: D

  Explanation: The molecule on the left is trans 1, 2-diiodocyclohexane. The molecule on the right is cis 1, 2-diiodocyclohexane. Cis and trans molecules are a type of diastereomer called geometric isomers.

  A: Structural isomers have fundamentally different connectivity between the atoms. So for example if the iodine were on carbons 1 and 3 and then in the other molecule on carbons 1 and 4, that would be structural isomers.

  C: Enantiomers are mirror image reflections of each other, which these molecules are not.

  D: Conformations isomers allow you to freely interconvert between the molecules without breaking any bonds. The classic example is chair and boat cyclohexane. Here, to flip the one iodine from R to S configuration would require breaking a bond in the ring, rotating, and then re-attaching. That requirement for bond-breaking means these are not conformers.

• Question 62:

  Mg(OH)2 is slowly dissolved in 500 mL of 25oC water until the solution becomes fully saturated. Which of the following occurs when 10.0 mL of 0.1 M HCl is added?

  A. MgCl2 precipitates.

  B. Mg(OH)2 precipitates.

  C. Ksp for Mg(OH)2 increases.

  D. [H2O] increases.

  Correct Answer: D

  This question is testing your understanding of solution chemistry, equilibrium, and solubility rules. In a saturated solution of Mg(OH)2, the maximum concentrations of Mg2+(aq) and OH-(aq) ions have been reached and are in equilibrium with

  the solubility product, as follows:

  Mg(OH)2(s) ?Mg2+(aq) + 2OH-(aq)

  When HCl is added to the solution, the pH of the solution decreases. The additional H+ ions react with the dissolved OH- ions forming water. Thus, D is the correct answer.

  Magnesium chloride is soluble in water, eliminating choice A.

  When HCl is added, the hydroxide concentration decreases shifting the equilibrium to the right, making choice B false.

  The solubility product, Ksp, is a constant that varies only with temperature, making choice C incorrect.

• Question 63:

  Which of the following best accounts for the negative slope of the liquid-solid equilibrium line in the phase diagram for water?

  A. H2O(s) has a greater density than H2O(l), which causes the solid to form liquid under high pressure conditions.

  B. H2O(s) has a greater density than H2O(l), which results from the hydrogen bonds formed between water molecules.

  C. H2O(s) has a lower density than H2O(l) which results from the crystalline framework that forms due to hydrogen bonds.

  D. H2O(s) has a lower density than H2O(l) which causes the solid to form liquid under low pressure conditions.

  Correct Answer: C

  This question asks the examinee to identify the correct explanation for the negatively sloped phase diagram for water. During the crystallization of water, each molecule forms four hydrogen bonds with adjacent water molecules resulting in three-dimensional hexagonal lattice structure. Thus, the density of solid water is less than the density of liquid water eliminating choices A and B. The negatively sloped liquid-solid equilibrium line in the phase diagram means that an increase in pressure at a constant temperature can cause water to change phases from solid to liquid. The increase in pressure causes water to convert to the more compact liquid phase in order to relieve the increased pressure. Therefore, answer choice C is the best answer.

• Question 64:

  If a gas occupies 0.1L at 200atm, what will its volume be at 1atm?

  A. slightly less than 20L

  B. 20L

  C. slightly more than 20L

  D. 2000L

  Correct Answer: A

  Ideal gases undergoing pressure and volume changes are related by the expression P1V1/T1=P2V2/T2. Here, temperature is not involved and P1V1= P2V2. However, since P1 is greater than 10atm, the size of the molecules cannot be

  ignored and volume V1 is slightly greater than it should be for an ideal gas. Therefore, the calculated volume V2=20L is slightly larger than it should be. Choice A is correct. B ?incorrect. The initial volume at a pressure of 200atm includes a

  small contribution from the molecules themselves, and therefore, the initial volume of 0.1L is larger than the volume of an ideal gas.

  C ?incorrect. See explanations for A and B.

  D ?incorrect. This answer choice incorrectly divides the initial pressure by the initial volume.

• Question 65:

  In an experiment to study frictional forces, a student attached a spring scale to identical bricks wrapped in different kinds of paper: brown paper, waxed paper and sand paper. Holding the spring scale, the student pulled the bricks across a table and measured the force required to pull the bricks across the table. The student wrapped the bricks in different paper to change the frictional forces between which two objects?

  A. The bricks and the spring scale.

  B. The paper and the bricks.

  C. The wrapped bricks and the table.

  D. None of these.

  Correct Answer: C

  Changing the type of wrapping paper would change the friction between the wrapped bricks and the table top. A greater frictional force would make it harder to pull the brick across the table and give a higher reading on the spring scale.

• Question 66:

  If KE =絤v2, express the unit of KE using the SI units of the terms in the equation.

  A. kg2 x m2 x s-2

  B. kg x m-2 x s2

  C. kg x m2 x s-2

  D. kg x m x s

  Correct Answer: C

  The SI unit of mass is kg and the SI unit of velocity is m.s-1 According to the equation, the SI unit of KE will be kg x m x s-1x m x s-1 First collect the terms as follows: kg x m x m x s-1x s-1 and then simplify, remembering to add the powers.

• Question 67:

  If (sin[0] + cos[180]) = x - 2(sin[90]) and all angles are in degrees, what is x?

  A. 0

  B. -1

  C. -2

  D. 1

  Correct Answer: D

  sin[0] = 0, sin[90] = 1 and cos[180] = -1. Substitute these values into the equation and solve for x:

  0 + (-1) = x - 2(1)

  -1 = x - 2

  x = 1

• Question 68:

  What happens to brain waves as an organism falls asleep?

  A. The frequency increases and amplitude increases.

  B. The frequency decreases and amplitude remains constant.

  C. The frequency increases and amplitude decreases.

  D. The frequency decreases and the amplitude increases.

  Correct Answer: D

  As an organism loses consciousness, bodily activity slows down and brain waves become slower and bigger.

• Question 69:

  A student observes that mercury forms a convex meniscus in the graduated cylinder but that water forms a concave one. This behavior is best explained by the fact that:

  A. the two liquids are being kept in graduated cylinders made of different materials.

  B. the adhesive forces between water and the walls of the graduated cylinder are greater than the adhesive forces between the mercury and the walls of the graduated cylinder.

  C. the cohesive forces between two mercury atoms are stronger than the cohesive forces between two water molecules.

  D. the mercury has strong cohesive than adhesive forces, whereas water has strong adhesive than cohesive ones.

  Correct Answer: D

  The shape of the meniscus lets you know whether the substance has more attractive force for itself (convex meniscus) or for the walls of the cylinder (concave meniscus). Water, for example, experiences more adhesion between the water and the walls of the container, thus "pulling" the water up the sides of the container creating a concave meniscus. The adhesive force is greater than the cohesive force between the water molecules. The opposite is true for mercury, thus making (D) the right answer.

  A: While this may be true, the student's observation doesn't lead to this conclusion.

  B, C: The shape of the meniscus doesn't let you compare the forces in water to the forces in mercury directly. It only lets you compare one force (adhesion) to another force (cohesion) for one substance.

• Question 70:

  What is the identity of the reducing agent and oxidizing agent, respectively, in the reaction shown below?

  2Cr(OH)3 + 3H2O2 + 4NaOH 2Na2CrO4 + 8H2O

  A. Cr(OH)+ and NaOH

  B. Cr(OH)3 and H2O2

  C. H2O2 and Cr(OH)3

  D. H2O2 and NaOH

  Correct Answer: B

  To answer this question, you must identify the atom that is reduced and the atom that is oxidized. In the reaction above, chromium is oxidized from +3 to +6, which makes chromium hydroxide the reducing agent. Alternatively, oxygen in hydrogen peroxide is reduced from -1 to -2, which makes it the oxidizing agent. Thus, the correct answer is B.