Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 151:

  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

  What is the approximate concentration of phosphoric acid in a 0.5 L sample of the beverage?

  A. 0.005 M

  B. 0.025 M

  C. 0.5 M

  D. 2.5 M

  Correct Answer: A

  Looking at the titration curve (Figure 1), it is clear that 500mL is added before the first endpoint of the titration (point B). The amount of base added is thus 500mL X 0.01mol/L = 0.005 mol. This quantity is the same as the amount of acid present initially. In other words, there is 0.005 mol of phosphoric acid initially present in the sample. In the second paragraph of the passage we are told that the sample has a volume of 1 L, and so the concentration of phosphoric acid in the sample is 0.005 mol/1 L = 0.005 M.The question asks for the concentration of a 0.5 L sample, but it does not make a difference: If we take 0.5 L from the 1-L sample, it will contain less phosphoric acid, but the concentration is the same.

• Question 152:

  The Earth's atmosphere reaches hundreds of kilometers above the surface of the planet. The lowest layer, the troposphere, extends from the ground to a height of approximately 12 km. Air pressure within the troposphere decreases with height above the ground, accompanied by a parallel trend in air density. The decrease in density has important consequences for the dissipation of air pollution from industrial smoke stacks. The gas from the stack is typically hotter and less dense than the surrounding air and rises. As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air. This results in a decrease in both its temperature and its density.

  

  Figure 1 A smoke stack functions to expel gaseous waste products from a chemical process. It is also an important means of removing heat from a reaction mixture. The heat corresponding to a change in temperature of a gas at constant pressure is

  

  given by , where is the heat added to the gas, n is the number of moles of gas, is the molar heat capacity of a particular gas at constant pressure, and T is the change in temperature. At atmospheric pressure, the

  

  molar heat capacity for steam, O (g) is approximately four times that of air.

  Two identical balloons of negligible mass are tethered at altitudes of 2000 meters and 2600 meters, respectively. The balloons are filled with helium gas to equal volumes. Which of the following statements is true concerning the buoyant force acting on each balloon?

  A. The buoyant force on the balloon at 2600 meters will be greater.

  B. The buoyant force on the balloon at 2000 meters will be greater.

  C. The buoyant forces on the two balloons will be equal.

  D. The relationship between the buoyant forces cannot be determined.

  Correct Answer: B

  The buoyant force acting on an object is equal in magnitude to the weight of the fluid it displaces. Because the balloons are of equal volume, they displace an equal volume of fluid. Lower altitude air is more dense, and thus, a given volume will weigh more. The balloon at the lower altitude (2000m) will have a greater buoyant force applied to it because the air is more dense at the lower altitude.This is similar to submerging an object in water, or in mercury which is much more dense. The buoyant force acting on the object when it is submerged in mercury will be much greater. Also, note that the question does not ask about the net force acting on the object, a calculation which would require us to determine the gravitational force.

• Question 153:

  The Earth's atmosphere reaches hundreds of kilometers above the surface of the planet. The lowest layer, the troposphere, extends from the ground to a height of approximately 12 km. Air pressure within the troposphere decreases with height above the ground, accompanied by a parallel trend in air density. The decrease in density has important consequences for the dissipation of air pollution from industrial smoke stacks. The gas from the stack is typically hotter and less dense than the surrounding air and rises. As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air. This results in a decrease in both its temperature and its density.

  

  Figure 1 A smoke stack functions to expel gaseous waste products from a chemical process. It is also an important means of removing heat from a reaction mixture. The heat corresponding to a change in temperature of a gas at constant pressure is

  

  given by , where is the heat added to the gas, n is the number of moles of gas, is the molar heat capacity of a particular gas at constant pressure, and T is the change in temperature. At atmospheric pressure, the

  

  molar heat capacity for steam, O (g) is approximately four times that of air.

  A combustion engine in a production plant is surrounded by pipes carrying water that function to cool the engine. The water is converted to steam and flows through a long vertical pipe to be released into the atmosphere. Heat is transferred from the engine to the atmosphere by the following means:

  A. convection then conduction

  B. radiation then convection then conduction

  C. conduction then convection then conduction

  Correct Answer: A

  Conduction is the transfer of heat between two objects in physical contact. Heat is conducted from the engine to the pipes by direct contact. Convection is the transfer of heat by movement of fluid. In this case, the flow of water and steam

  carries the heat away from the engine and into the atmosphere. Radiation is the transfer of heat without contact, through thermal radiation. The sun transfers heat to the earth by radiation.

  Choice B is incorrect because the heat is first transferred by conduction.

  Choice C is incorrect. The question does not specifically state that the water pipes are in direct contact with the engine. It could be supposed that the engine radiates heat to the pipes, which then carry the heat away by convection. However,

  there is no conduction following the transfer of heat to the atmosphere by convection, and so this answer is incorrect.

  Choice D is incorrect because there is no conduction following the transfer of heat by convection.

• Question 154:

  The Earth's atmosphere reaches hundreds of kilometers above the surface of the planet. The lowest layer, the troposphere, extends from the ground to a height of approximately 12 km. Air pressure within the troposphere decreases with height above the ground, accompanied by a parallel trend in air density. The decrease in density has important consequences for the dissipation of air pollution from industrial smoke stacks. The gas from the stack is typically hotter and less dense than the surrounding air and rises. As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air. This results in a decrease in both its temperature and its density.

  

  Figure 1 A smoke stack functions to expel gaseous waste products from a chemical process. It is also an important means of removing heat from a reaction mixture. The heat corresponding to a change in temperature of a gas at constant pressure is

  

  given by , where is the heat added to the gas, n is the number of moles of gas, is the molar heat capacity of a particular gas at constant pressure, and T is the change in temperature. At atmospheric pressure, the

  

  molar heat capacity for steam, O (g) is approximately four times that of air.

  The decrease in temperature of a parcel of hot air rising from a smoke stack is a result of:

  A. conductive heat losses to the surrounding cooler air.

  B. convective heat losses to the surrounding cooler air.

  C. an increase in kinetic energy.

  D. the expansion of the parcel.

  Correct Answer: D

  The passage states that "As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air". Adiabatic processes have no transfer of heat. This eliminates Choices A and B. Choice C is incorrect because the kinetic energy decreases as temperature decreases.

• Question 155:

  The Earth's atmosphere reaches hundreds of kilometers above the surface of the planet. The lowest layer, the troposphere, extends from the ground to a height of approximately 12 km. Air pressure within the troposphere decreases with height above the ground, accompanied by a parallel trend in air density. The decrease in density has important consequences for the dissipation of air pollution from industrial smoke stacks. The gas from the stack is typically hotter and less dense than the surrounding air and rises. As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air. This results in a decrease in both its temperature and its density.

  

  Figure 1 A smoke stack functions to expel gaseous waste products from a chemical process. It is also an important means of removing heat from a reaction mixture. The heat corresponding to a change in temperature of a gas at constant pressure is

  

  given by , where is the heat added to the gas, n is the number of moles of gas, is the molar heat capacity of a particular gas at constant pressure, and T is the change in temperature. At atmospheric pressure, the

  

  molar heat capacity for steam, O (g) is approximately four times that of air.

  Ozone molecules in the stratosphere absorb ultraviolet radiation and photodissociate into an oxygen molecule (O2), and a free oxygen atom (O). Which of the following best describes this phenomenon?

  A. The wavelength of the ultraviolet radiation is greater than the cutoff wavelength needed for photodissociation.

  B. The frequency of the ultraviolet radiation is greater than the cutoff frequency needed for photodissociation.

  C. The frequency of the ultraviolet radiation is less than the cutoff frequency needed for photodissociation.

  D. The wavelength of the ultraviolet light is less than the cutoff frequency.

  Correct Answer: B

  This question requires an understanding of the energy of photons. The energy is related to the frequency by the following equation, where h is Planck's constant:

  

  Higher energy photons have a higher frequency, and shorter wavelength. The frequency is inversely related to the wavelength according to the following equation:

  

  Thus, the cutoff energy for photodissociation would be related to a minimum frequency (or a maximum wavelength) of light.

• Question 156:

  The Earth's atmosphere reaches hundreds of kilometers above the surface of the planet. The lowest layer, the troposphere, extends from the ground to a height of approximately 12 km. Air pressure within the troposphere decreases with height above the ground, accompanied by a parallel trend in air density. The decrease in density has important consequences for the dissipation of air pollution from industrial smoke stacks. The gas from the stack is typically hotter and less dense than the surrounding air and rises. As a parcel of hot air rises, it expands approximately adiabatically doing work on the surrounding air. This results in a decrease in both its temperature and its density.

  

  Figure 1 A smoke stack functions to expel gaseous waste products from a chemical process. It is also an important means of removing heat from a reaction mixture. The heat corresponding to a change in temperature of a gas at constant pressure is

  

  given by , where is the heat added to the gas, n is the number of moles of gas, is the molar heat capacity of a particular gas at constant pressure, and T is the change in temperature. At atmospheric pressure, the

  

  molar heat capacity for steam, O (g) is approximately four times that of air.

  According to Figure 1, what is the approximate relationship between the energy content of a liter of air at 12 km and that of a liter of air at 120 km?

  A. The energy content in a liter at 12 km is greater because the mass is greater.

  B. The energy content in a liter at 120 km is greater because the temperature is higher.

  C. The energy content in a liter at 120 km is greater because the air is less dense.

  D. The energy contents are equal because the volumes are equal.

  Correct Answer: A

  Looking at Figure 1, we see that the temperature at 120 km is significantly higher than at 12 km.

  

  However, the graph also indicates that the pressure at 120 km is essentially zero. The passage indicates that as the pressure decreases, so does the density. Thus, the mass of a liter of air at this altitude is near zero and the energy content of the air is minimal, regardless of the higher temperature. You may also rearrange the ideal gas law to obtain:

  

  which tells you that as P approaches zero, so does n/V, which is an indication of density.

• Question 157:

  A hovercraft is a versatile vehicle capable of traveling over land, water, or any other essentially flat surface. The hovercraft consists of a body or hull onto which a rotor (lift fan) is mounted. The lift fan provides the vertical lift by propelling air into an area beneath the hovercraft called the skirt. The pocket of air in the skirt supports the moving hovercraft and reduces the friction between the vehicle and the ground to almost zero. As such, there is no contact between the hovercraft and the ground.

  A second fan, which generates a horizontal thrust, propels the hovercraft forward. Rudders which direct the airflow from this second fan are used by the pilot to control the movement of the hovercraft. The horizontal movement of the

  

  hovercraft is opposed by air resistance which generates aerodynamic drag.

  The thrust fan of a hovercraft is turned on at time t1 and turned off at time t2. In the absence of air resistance, which of the following graphs represents the velocity of this hovercraft as a function of time?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  The thrust fan provides the force which propels the hovercraft forward. In the absence of wind resistance, it is the only (and hence net) force acting upon the hovercraft. From the time t1 that the fan is turned on until time t2, the hovercraft will experience an acceleration supplied by the thrust in accordance with Newton's second law. At the moment t2 that the fan is turned off, the hovercraft experiences no net force and therefore no acceleration. According to Newton's first law, in the absence of a net force the hovercraft will continue in motion with constant velocity. The increasing velocity due to a constant acceleration between t1 and t2 is represented by the positively sloped line in graph A. The constant velocity after the fan is turned off is represented by the horizontal line of graph A.Choice B is incorrect because in the absence of air resistance, the velocity of the hovercraft would not decrease after the thrust has been turned off at time t2. Choice C is incorrect because according to the graph, the hovercraft undergoes constant acceleration even after t2 This cannot be true, as after t2 the hovercraft experiences no net force and thus no acceleration. Choice D is incorrect as the graph shows an instantaneous change in velocity which would require an infinite acceleration.

• Question 158:

  A hovercraft is a versatile vehicle capable of traveling over land, water, or any other essentially flat surface. The hovercraft consists of a body or hull onto which a rotor (lift fan) is mounted. The lift fan provides the vertical lift by propelling air into an area beneath the hovercraft called the skirt. The pocket of air in the skirt supports the moving hovercraft and reduces the friction between the vehicle and the ground to almost zero. As such, there is no contact between the hovercraft and the ground.

  A second fan, which generates a horizontal thrust, propels the hovercraft forward. Rudders which direct the airflow from this second fan are used by the pilot to control the movement of the hovercraft. The horizontal movement of the

  

  hovercraft is opposed by air resistance which generates aerodynamic drag.

  Which of the following conditions must be true for the thrust fan to propel the hovercraft horizontally? (Assume that the air is not compressed in the process.)

  A. The air intake velocity must be lower than the air output velocity.

  B. The air intake velocity must be greater than the air output velocity.

  C. The air intake volume must be lower than the air output volume.

  D. The air intake pressure must be greater than the air output pressure.

  Correct Answer: A

  Using the continuity equation it is clear that the volume of air entering the fan must equal the volume of air leaving the fan. The air is accelerated and so its velocity is increased. Conservation of momentum shows why propelling air backwards

  at a high velocity will move the hovercraft forward.Choice B is incorrect because the air intake velocity must be lower than the output velocity. The velocity is increased by the fan.Choice C is incorrect because the intake volume is equal to the

  output volume since no compression of the air takes place.

  Choice D is incorrect because the intake pressure is equal to the output pressure by the continuity equation.

• Question 159:

  A hovercraft is a versatile vehicle capable of traveling over land, water, or any other essentially flat surface. The hovercraft consists of a body or hull onto which a rotor (lift fan) is mounted. The lift fan provides the vertical lift by propelling air into an area beneath the hovercraft called the skirt. The pocket of air in the skirt supports the moving hovercraft and reduces the friction between the vehicle and the ground to almost zero. As such, there is no contact between the hovercraft and the ground.

  A second fan, which generates a horizontal thrust, propels the hovercraft forward. Rudders which direct the airflow from this second fan are used by the pilot to control the movement of the hovercraft. The horizontal movement of the

  

  hovercraft is opposed by air resistance which generates aerodynamic drag.

  The horizontal thrust fan of a hovercraft traveling across a frozen lake in a large circle suddenly fails. Which of the following best describes the path of the hovercraft (as one looks down from above) immediately following thrust fan shutoff?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  When the thrust fan is shut off, there will no longer be a centripetal force to sustain the hovercraft in its circular motion. Thus, it will proceed in a straight line tangent to its path at the moment the fan is turned off.Choice B is incorrect because a force (centripetal force) is required to cause an object to move in a circle. No such force is being applied to the hovercraft after the thrust fan is shut off.Choice C is incorrect. This arrow indicates the direction of a centripetal force required to keep an object moving in a circle. This would be the force generated by the thrust fan before it is shut off.Choice D is incorrect because a force directed out of the circle would be required to cause the hovercraft to follow this path. No such force exists in this scenario.

• Question 160:

  A hovercraft is a versatile vehicle capable of traveling over land, water, or any other essentially flat surface. The hovercraft consists of a body or hull onto which a rotor (lift fan) is mounted. The lift fan provides the vertical lift by propelling air into an area beneath the hovercraft called the skirt. The pocket of air in the skirt supports the moving hovercraft and reduces the friction between the vehicle and the ground to almost zero. As such, there is no contact between the hovercraft and the ground.

  A second fan, which generates a horizontal thrust, propels the hovercraft forward. Rudders which direct the airflow from this second fan are used by the pilot to control the movement of the hovercraft. The horizontal movement of the

  

  hovercraft is opposed by air resistance which generates aerodynamic drag.

  A 600 kg hovercraft traveling across a frozen lake accelerates from rest to a speed of 60 kilometers per hour in 4 seconds. What is the average force produced by the thrust fan?

  A. 4.2 N

  B. 145 N

  C. 2500 N

  D. 9000 N

  Correct Answer: C

  Explanation: This problem can be solved very quickly using dimensional analysis.

  

  We can also solve it the longer way. The final velocity of the hovercraft in units of m/s is:

  

  The average acceleration is: The force can be determined using Newton's Second Law: