Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 171:

  The nuclei of certain unstable isotopes will spontaneously decay, producing a more stable nucleus and releasing a particle or quantity of energy. Alpha decay releases a helium nucleus, beta decay emits an electron, while gamma decay is the emission of a high energy photon. Each type of radioactive decay is characterized, in part, by the half-life of the radioactive material--the time required for half of the nuclei in a sample to undergo decay. Examples of such decays are shown in Figure 1.

  

  Figure 1

  A Geiger counter can be used to detect the decay of radioactive materials. A simple Geiger counter consists of a hollow metal cylinder with a wire along its axis. The cylinder is filled with low pressure argon gas and a high voltage difference is

  applied between the wire and the cylinder. When alpha, beta, or gamma radiation passes through the cylinder, it interacts with the gas particles and leads to the formation of ions which cause a discharge between the wire and the cylinder.

  The consequent current may be used to drive a speaker, producing the characteristic clicking sound of the Geiger counter each time a pulse of current occurs. The Geiger counter circuitry is shown in Figure 2.

  

  Figure 2

  Assume that the values for V and R are known. What other information is needed to calculate the voltage drop across the speaker?

  A. The resistance of the speaker's internal circuitry.

  B. The duration of the discharge between the wire and the cylinder.

  C. The current flowing through the resistor.

  D. No other information is required.

  Correct Answer: C

  Explanation: The circuit diagram indicates that the resistor and speaker are wired in parallel. Therefore, the voltage across the resistor is the same as the voltage across the speaker. We can calculate the voltage across the resistor using V = IR. The only additional information we need is the value of I, the current flowing through the resistor.Choice A is incorrect. We need to know the current through the speaker, as well as its internal resistance, to determine the voltage across the speaker. The resistance alone is insufficient.Choice B is incorrect because the duration of the discharge would not provide us with any useful information. If we knew the charge transmitted and the duration, we could calculate the current. The duration alone is insufficient.Choice D is incorrect because other information is required, as described above.

• Question 172:

  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 )

  

  The pressure ( ) in the first chamber is decreased by raising a piston and increasing the volume of the chamber from to . If the temperature is kept at 160 °C, which of the following values solves for the new pressure ( ) in the chamber?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  

  Using Boyle's Law

• Question 173:

  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 would be most likely to occur if the second chamber was kept at 50?C instead of 0?C?

  A. The water vapor would not condense.

  B. Liquid water would be able to travel through the tube between chambers.

  C. A different polymer of polywater would form.

  D. The water vapor would condense more slowly.

  Correct Answer: D

  The water would condense less rapidly because the rate of heat transfer from the water vapor to the atmosphere would be decreased. Heat is transferred from a body of high temperature to one of lower temperature. This is a consequence of the second law of thermodynamics. The rate of heat transfer depends on the temperature gradient.Choice A is incorrect because the water would condense. The boiling point of water is 100°C, so elevating the temperature to 50°C would not stop water condensation.Choice B is incorrect because the temperature of the tube (500°C) has not been changed and only water vapor will pass through. Liquid water will be vaporized.Choice C is incorrect because there the passage indicates that polywater never formed (Hypothesis 2). Furthermore, there is no evidence that altering the temperature would change the structure of polywater.

• Question 174:

  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.

  Correct Answer: A

  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 175:

  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 changes in the experimental apparatus would increase the rate of water vapor production in the first chamber of the experimental apparatus? (All other conditions kept constant.)

  A. Decreasing the temperature

  B. Decreasing the pressure

  C. Decreasing the cross-sectional area of the tube connecting the two chambers

  D. Adding NaCl to the distilled water in the first chamber

  Correct Answer: B

  Decreasing the pressure would increase the water vapor production. Looking at the equilibrium below, it is clear that reduction of pressure would favor the formation of gas by Le Chatelier's principle.

  

  Choice A is incorrect. Decreasing the temperature would decrease water vapor production. Note that adding heat to the equilibrium shown above would drive the reaction to the right. Conversely, removing heat would drive the reaction to the

  left.

  Choice C is incorrect because decreasing the cross-sectional area of the tube, if anything, would slow down the rate of vapor production because the vapor would not escape the first chamber as rapidly.

  Choice D is incorrect because increasing the solute concentration of the water would tend to decrease the vapor pressure.

• Question 176:

  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 )

  Assume that Hypothesis 2 is correct. Compared to normal water, polywater would have a:

  A. lower vapor pressure and higher boiling point.

  B. lower vapor pressure and lower boiling point.

  C. higher vapor pressure and lower boiling point.

  D. similar vapor pressure and boiling point.

  Correct Answer: A

  Vapor pressure and boiling point are colligative properties that depend on the number of particles in a solution. Assuming that hypothesis 2 is correct and that polywater is merely an aqueous solution with dissolved materials, vapor pressure depression and boiling point elevation would occur.

• Question 177:

  A5-kg mass M is being raised from the ground to the top of the inclined plane using the set-up shown in the diagram below. Assuming that the inclined plane is frictionless, what is the work done by the force F?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  There are two ways to solve this problem. The fast, easy way is to conduct a work-energy analysis. The initial energy of the mass is taken as 0. The final energy (at the top of the incline) is mgh = 5(10)(10) = 500J. This change in energy is equal to the work done on the mass. We could also calculate the F required to lift the box and the distance over which that force is applied.

  

  The force required to move the box is equal to The distance travelled by the box up the incline is given by the pythagorean equation. Distance =

  

• Question 178:

  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 )

  

  Hypothesis 1 proposes that polywater is a repeating structure of monomers. What is the formal charge on hydrogen in this structure?

  A. -1

  B. +1

  C. -2

  D. +2

  Correct Answer: A

  The formal charge on an atom is given by the following equation:Formal charge = valence electrons in neutral atom ?non-bonding electrons ?1/2 bonding electrons Formal charge H = 1 ?0 ?2 = ?

• Question 179:

  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 ) According to the passage, polywater has all of the following properties EXCEPT:

  A. light entering polywater from air will bend more towards the normal than light entering normal water.

  B. polywater can be produced in a variety of conditions.

  C. 1 g of water has a larger volume than 1 g of polywater, at room temperature.

  D. polywater exists in the liquid state at 0?C.

  Correct Answer: B

  The passage describes a very intricate mechanism for producing polywater and does not support the fact that polywater can be produced in a variety of conditions. Choice A is incorrect because polywater has a higher index of refraction than normal water and so would bend light more towards thenormal (smaller angle).

  

  Choice C is incorrect because polywater is 40% more dense than water. So, an equal mass of polywater will have a smaller volume.Choice D is incorrect because the passage states that polywater has a freezing-point of ?0?C.

• Question 180:

  A student conducts a chemical analysis of the components of a popular soft drink. The beverage label shows that the drink contains carbonated water, phosphoric acid, caffeine, and caramel color, but does not indicate the concentrations of these chemicals.

  

  Dissolved carbon dioxide will react reversibly with water to form carbonic acid. In an attempt to analyze the beverage composition, the student conducts the following experiments on a one liter sample of the beverage.

  Experiment 1

  The sample is placed in a sealed beaker cooled to 10?C and a vacuum is created in the space above the beverage. The gas pumped from this space is passed through a solution of BaCl2, producing a white precipitate. The process

  continues until no more precipitate forms. The precipitate is dried and found to have a mass of 9.5 grams.

  Experiment 2

  The remaining solution left in the sealed beaker is then titrated with 0.01 M NaOH to give the titration curve shown in Figure 1.

  

  Figure 1

  

  Why did the student choose to keep the sealed beaker of beverage at 10?C while vacuuming the from solution?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The beverage is kept at a low temperature to reduce its kinetic energy and minimize the loss of water by evaporation. If too much water were lost, the concentration of the remaining fluid would increase and inaccurate measurements of the

  phosphoric acid concentration would result. Choice A is incorrect because CO2 is a non-polar molecule and would not be involved in dipole-dipole interactions with water.

  Choice B is incorrect because extracting the gas from the beverage would not result in a violent expansion of the gas.

  Choice D is incorrect because reducing the temperature would decrease the kinetic energy.