Medical Tests MCAT-TEST Online Practice Questions and Exam Preparation

Medical Tests MCAT-TEST Online Questions & Answers

• Question 11:

  The reaction R -- Br + Br*? R -- Br* + Br -is always accompanied by inversion. If this reaction is carried out on an optically pure sample of a chiral compound, which of the following statements will be true? [Note: Br* represents a radioactive isotope of bromine.]

  A. The rate of Br* incorporation is half the rate of racemization.
  B. The rate of Br* incorporation is equal to the rate of racemization.
  C. The rate of Br* incorporation is twice the rate of racemization.
  D. The relation between the rate of Br* incorporation and the rate of racemization cannot be determined.
  A. The rate of Br* incorporation is half the rate of racemization.

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  Explanation/Reference:

  This is another chemistry question about SN2 reactions and their effect on optical activity. Here you have to think about exactly what's happening as the stated reaction proceeds. Each time an alkyl halide molecule reacts, incorporating a radioactive bromine atom, it will be optically inverted; that is, it will go from R to S or vice versa. Each molecule thus inverted "cancels out" one of the molecules that still has the old configuration; thus each inversion event leads to the racemization of two molecules of alkyl halide. So the rate of radioactive bromine incorporation is equal to half of the rate of racemization, and so the correct answer is A.

• Question 12:

  How will the gravitational force between two objects change if the distance between them is doubled?

  A. It will remain constant.
  B. It will decrease by a factor of 4.
  C. It will increase by a factor of 4.
  D. It will be cut in half.
  B. It will decrease by a factor of 4.

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  Explanation/Reference:

  The gravitational force between two objects is inversely proportional to the distance between them squared.

• Question 13:

  The equation of state of an ideal gas is given by the ideal gas law:

  PV = nRT

  where P is the pressure, V is the volume, n is the number of moles of gas, R is the ideal gas constant, and T is the temperature of the gas. The gas particles in a container are constantly moving at various speeds. These speeds are

  characterized by the Maxwell shown in the figure below.

  

  If two particles collide, their velocities change. However, if the gas is in thermal equilibrium, the velocity distribution of the gas as a whole will remain unchanged by the collision. The average kinetic energy (E) of a gas particle is given by:

  

  Equation 1

  where m is the mass of one particle and u is the root mean square speed (rms speed) of the gas particles:

  

  where N is the number of gas particles; this is different from the average speed). For an ideal gas, the kinetic energy of all the particles is:

  

  Equation 2

  where n is the number of moles of gas. Combining these equations gives:

  

  Equation 3

  where M is the molar mass of the gas particles.

  The average distance a particle travels between collisions is known as the mean free path l. Intuitively, the mean free path (mfp) could be expected to be larger for gases at low pressure, since there is a lot of space between particles.

  Similarly, the mfp should be larger when the gas particles are small. The following expression for the mfp shows this to be correct.

  

  Equation 4 In this equation, s is the atomic diameter (typically on the order of 10?), k is the Boltzmann constant, and P is the pressure. In addition to colliding with one another, gas particles also collide with the walls of their container. If the container wall has a pinhole that is small compared to the mfp of the gas, and a pressure differential exists across the wall, the particles will effuse (or escape) through this pinhole without disturbing the Maxwellian distribution of the particles. The rate of effusion can be described by:

  

  Equation 5

  Where neff is the number of moles of effusing particles, A is the area of the pinhole, p and p1 are the pressures on the inside and outside of the container wall respectively, and p>p1.

  Which of the following will have the smallest root mean square speed at 298K?

  

  A. Option A
  B. Option B
  C. Option C
  D. Option D
  A. Option A

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  Explanation/Reference:

  

  

  Equation 3 states that the root mean square speed, u, is equal to (3RT/M)? Therefore, u is proportional to . Since the temperature is held constant, we must look at the molar mass. If u is proportional to the molecule with the largest molar mass will have the smallest root mean square speed. Chlorine, choice A, has the largest molar mass (71 g/mol) and is the correct response. Oxygen, carbon dioxide, and nitrogen have molar masses of 32 g/mol, 44 g/ mol, and 28 g/mol, respectively. When compared with chlorine, we can see that all of these molecules will have a larger root mean square speed.

• Question 14:

  Due to ever-increasing paranoia about the transmission of hepatitis and AIDS via blood transfusions and the frequent difficulty of procuring matching blood donors for patients, researchers have been working at a feverish pace to produce disease-free and easy-to-use blood substitutes. The difficulty most synthetic blood researches have had is in formulating a substance that combines qualities of sterility, high capacity for carrying oxygen to body tissues, and versatility within the human body. Three major substitute technologies have been developed to date; each has certain advantages and shortcomings.

  "Red blood," the first of the blood substitute technologies, is derived from hemoglobin which has been recycled from old, dead, or worn-out red blood cells and modified so that it can carry oxygen outside the red blood cell. Hemoglobin, a complex protein, is the blood's natural oxygen carrier and is attractive to scientists for use in synthetic blood because of its oxygen-carrying capacity. However, hemoglobin can sometimes constitute a two-fold threat to humans when it is extracted from the red blood cell and introduced to the body in its naked form. First, hemoglobin molecules are rarely sterile and often remain contaminated by viruses to which they were exposed in the cell. Second, naked hemoglobin is extremely dangerous to the kidneys, causing blood flow at these organs to shut down and leading, ultimately, to renal failure. Additional problems arise from the fact that hemoglobin is adapted to operate optimally within the intricate environment of the red blood cell. Stripped of the protection of the cell, the hemoglobin molecule tends to suffer breakdown within several hours. Although modification has produced more durable hemoglobin molecules which do not cause renal failure, undesired side effects continue to plague patients and hinder the development of hemoglobin-based blood substitutes.

  Another synthetic blood alternative, "white blood", is dependent on laboratory synthesized chemicals called perfluorocarbons (PFCs). Unlike blood, PFCs are clear oil like liquids, yet they are capable of absorbing quantities of oxygen up to 50% of their volume, enough of an oxygen carrying potential for oxygen-dependent organisms to survive submerged in the liquid for hours by "breathing" it. Although PFCs imitate real blood by effectively absorbing oxygen, scientists are primarily interested in them as constituents of blood substitutes because they are inherently safer to use than hemoglobin-based substitutes. PFCs do not interact with any chemicals in the body and can be manufactured in near-perfect sterility. The primary pitfall of PFCs is in their tendency to form globules in plasma that can block circulation. Dissolving PFCs in solution can mitigate globulation; however, this procedure also seriously curtails the PFCs' oxygen capacity.

  The final and perhaps most ambitious attempt to form a blood substitute involves the synthesis of a modified version of human hemoglobin by genetically-altered bacteria. Fortunately, this synthetic hemoglobin seems to closely mimic the qualities of sterility, and durability outside the cellular environment, and the oxygen-carrying efficiency of blood. Furthermore, researchers have found that if modified hemoglobin genes are added to bacterial DNA, the bacteria will produce the desired product in copious quantities. This procedure is extremely challenging, however, because it requires the isolation of the human gene for the production of hemoglobin, and the modification of the gene to express a molecule that works without support from a living cell. While all the above technologies have serious drawbacks and difficulties, work to perfect an ideal blood substitute continues. Scientists hope that in the near future safe synthetic blood transfusions may ease blood shortages and resolve the unavailability of various blood types.

  Which of the following is mentioned in the passage as a problem specific to "red blood"?

  A. "Red blood" cannot be produced in large enough quantities.
  B. "Red blood" tends to form globules that block circulation.
  C. Hemoglobin does not carry oxygen effectively.
  D. "Red blood" exhibits poor durability in the bloodstream.
  D. "Red blood" exhibits poor durability in the bloodstream.

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  Explanation/Reference:

  This is a detail question that requires you to recall which of the answer choices is a problem specific to "red blood". Since "red blood" is discussed in the second paragraph of the passage, refer there for details. Choice (A) is incorrect because the passage never says that "red blood cannot be produced in large quantities". In fact, the passage never at all mentions the extent to which "red blood" can be produced. The passage only mentions quantities of production of synthetic blood in the third sentence of the fourth paragraph, where it states, in reference to genetically- altered hemoglobin, that "the bacteria will produce the desired product in copious quantities". Since the passage never mentions (A), it cannot be a problem specific to "red blood". Similarly, choice (B) distorts a detail of the passage which relates to another type of synthetic blood. The passage mentions in the fourth sentence of the third paragraph that the primary pitfall of PFCs is their tendency to form globules. The passage never says that "red blood'' forms globules. Choice (C) also is not a true statement. The second sentence of the second paragraph states that hemoglobin is the blood's natural oxygen carrier and, in fact, is attractive to scientists precisely because of its oxygen-carrying capacity. You may safely conclude from this information, then, that hemoglobin does, indeed, carry oxygen effectively and (C) is not a problem at all. Only choice (D), which states that "red blood exhibits poor durability in the bloodstream", is mentioned in the passage as a problem specific to "red blood". In the second paragraph, the passage states that "additional problems arise from the fact that hemoglobin is adapted to operate optimally within the...cell. Stripped of the protection of the cell, the hemoglobin molecule tends to suffer breakdown within several hours". One of the criteria for a successful blood substitute is that it be durable and versatile. The fact that "red blood" breaks down readily in the bloodstream shows that "red blood" is not durable in the bloodstream. This is the problem mentioned specifically in relation to "red blood", so choice (D) is the correct answer.

• Question 15:

  If volume of a given mass of a gas is doubled without changing its temperature, the density of the gas:

  A. is doubled.
  B. remains unchanged.
  C. is halved.
  D. is reduced to 1/4 of the original value.
  B. remains unchanged.

• Question 16:

  The frequency of second's pendulum is:

  A. 2 Hertz
  B. 0.5 Hertz
  C. 1 Hertz
  D. 0.25 Hertz
  B. 0.5 Hertz

• Question 17:

  In 1965, Boris Deryagin reported the discovery of an unusual substance formed during the condensation of water vapor in quartz capillaries. The material, called poly-water, appeared to be a polymer of water monomers and differed from normal water in a number of ways. It had a freezing point of ?0?C and solidified into a glass-like solid with substantially less volumetric expansion than that of ordinary water upon freezing. It had a density 40% greater than water and a refractive index of 1.48. An intricate apparatus was used to produce the poly-water. Ordinary distilled water was placed in a chamber held at 160?C with pressure below atmospheric pressure. This chamber was connected to a second chamber by a tube held at 500?C in order to prevent the passage of liquid water. The second chamber was held at 0?C and contained a drawn quartz capillary in which the water vapor condensed, forming poly-water.

  

  Hypothesis 1

  Deryagin proposed that polywater was a polymer of water monomers arranged in a network of hexagonal units. The polymerization was catalyzed by the silicate surface of the quartz capillary.

  

  Proposed Structure of Polywater Hypothesis 2

  

  Another researcher was skeptical. Analysis indicated that polywater was merely a solution of water and dissolved particles including silicon, carbon dioxide, and substantial concentrations of ions These contaminants dissolved from the quartz capillary and from materials used in the apparatus.

  

  (constants for normal water : density = 1 g/c , index of refraction = 1.33 , freezing point depression constant = 1.86°C )

  Which of the following pieces of evidence would most support Hypothesis 1?

  A. The mass of the quartz capillary did not change throughout the experiment.
  B. Filtration of the polywater increased its freezing temperature.
  C. The polywater was found to differ from normal water in its boiling point.
  D. The second chamber could be kept at 50?C with similar results.
  A. The mass of the quartz capillary did not change throughout the experiment.

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  Explanation/Reference:

  If the mass of the capillary was unchanged, this would weaken Hypothesis 2 which states that the contaminants in polywater dissolved from the quartz capillary. Weakening Hypothesis 2 would support Hypothesis 1.Choice B is incorrect. If de-ionization of polywater increased its freezing temperature, this would indicate that polywater contained dissolved solids and provide support for Hypothesis 2.Choice C is incorrect. If Hypothesis 2 is correct, we expect polywater to differ from normal water in its boiling point. Polywater would then have a higher boiling point.Choice D is incorrect because this would neither support nor weaken Hypothesis 1.

• Question 18:

  The simple harmonic motion of a mass suspended from vertical springs is investigated in two experiments. The springs used in both experiments have a spring constant k and a natural length L0. The material used to make the springs has a

  Young's modulus of 2 x 1011 Pa.

  In the first experiment a mass m is suspended from a spring. The mass stretches the spring to a new length L, called the equilibrium length.

  In the second experiment the mass m is suspended from two identical springs as shown in Figure 2 below. When the mass m is in equilibrium, each spring is stretched from its natural length by the same amount xe.

  In both experiments the masses of the springs are negligible, and the elastic limits of the springs are never exceeded.

  

  The mass in the first experiment is pulled down a distance A from its equilibrium position and then released from rest. The mass will then oscillate with simple harmonic motion. As the mass moves up and down, energy is dissipated due to factors such as air resistance and internal heating of the spring. The mass will no longer oscillate when the total energy dissipated equals:

  A. kL2/2
  B. kA2/2
  C. k(L + A)2/2
  D. kL0 2/2
  B. kA2/2

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  Explanation/Reference:

• Question 19:

  How much heat energy is required when 8 grams of hydrogen are burnt? The thermal reaction is:

  2H2 + O2 2H2O + 136.64 kcal

  A. 273.56 kcal
  B. 68.32 kcal
  C. 136.64 kcal
  D. 102.48 kcal
  A. 273.56 kcal

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  Explanation/Reference:

• Question 20:

  The polymerase chain reaction (PCR) is a powerful biological tool that allows the rapid amplification of any fragment of DNA without purification. In PCR, DNA primers are made to flank the specific DNA sequence to be amplified. These primers are then extended to the end of the DNA molecule with the use of a heat- resistant DNA polymerase. The newly synthesized DNA strand is then used as the template to undergo another round of replication.

  The 1st step in PCR is the melting of the target DNA into 2 single strands by heating the reaction mixture to approximately 94 oC, and then rapidly cooling the mixture to allow annealing of the DNA primers to their specific locations. Once the primer has annealed, the temperature is elevated to 72 oC to allow optimal activity of the DNA polymerase. The polymerase will continue to add nucleotides until the entire complimentary strand of the template is completed at which point the cycle is repeated (Figure 1)

  

  Figure 1

  One of the uses of PCR is sex determination, which requires amplification of intron 1 of the amelogenin gene. This gene found on the X-Y homologous chromosomes has a 184 base pair deletion on the Y homologue. Therefore, by amplifying intron 1 females can be distinguished from males by the fact that males will have 2 different sizes of the amplified DNA while females will only have 1 unique fragment size.

  The polymerase chain reaction most likely resembles which of the following cellular process?

  A. Transcription of DNA
  B. Protein synthesis
  C. DNA replication
  D. Translation
  C. DNA replication

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  Explanation/Reference:

  As described in the passage, the polymerase chain reaction utilizes DNA primers that anneal to the DNA template. DNA polymerase then extends the DNA primers in an effort to replicate the DNA strand. These steps are identical to those of DNA replication in a cell. However, in a cell, the cycle occurs once per cellular division.