Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 271:

  It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pH are called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. The table below lists the major buffers found in the blood and/or kidneys. Table 1 Buffer pKa of a typical conjugate acid:*

  

  + Histidine side chains

  

  Organic phosphates N-terminal amino groups

  

  6.1

  6.3

  6.8

  7.0

  8.0

  9.2

  

  *For buffers in many of these categories, there is a range of actual values.

  

  The relationship between blood pH and the of any buffer can be described by the Henderson-Hasselbalch equation:

  

  pH = + log([conjugate base]/[conjugate acid]) Equation 1

  

  Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, a byproduct of metabolism, and leaves in two forms: exhaled and excreted bicarbonate. Blood pH can be adjusted rapidly by changes

  

  in the rate of exhalation. The reaction given below, which is catalyzed by carbonic anhydrase in the erythrocytes, describes how bicarbonate and interact in the blood.

  

  + + Reaction 1

  

  The following graph shows the titration of 0.01 M with 10 M NaOH. Within which region of the titration curve will the concentration of become equal to that of ?

  

  A. II

  B. III

  C. IV

  D. V

  Correct Answer: B

  In Region I the following reaction takes place: H3PO4(aq) + OH?(aq) H2PO4?(aq) + H2O.

  As OH?is added, H2PO4?is formed and a buffer is realized. So, in this region, there is a point where the concentration of H3PO4 will equal that of H2PO4? In Region II there is a point where exactly enough base has been added to react with

  all of the H3PO4; this point is called an equivalence point. In Region III the following reaction takes place: H2PO4?(aq) + OH?aq) HPO42- (aq) + H2O. Again, this system constitutes a buffer, and there is a point in this region where--after

  enough base has been added--the concentration of H2PO4?will equal that of HPO42-. Choice B is therefore the correct response. In Region IV there is another equivalence point when exactly enough base has been added to react with all of

  the H2PO4? In Region V the following reaction takes place: HPO42- (aq) + OH?aq) PO43- (aq) + H2O.

  This system is, of course, a buffer as well.

• Question 272:

  It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pH are called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. The table below lists the major buffers found in the blood and/or kidneys. Table 1 Buffer pKa of a typical conjugate acid:*

  

  + Histidine side chains

  

  Organic phosphates N-terminal amino groups

  

  6.1

  6.3

  6.8

  7.0

  8.0

  9.2

  

  *For buffers in many of these categories, there is a range of actual values.

  

  The relationship between blood pH and the of any buffer can be described by the Henderson-Hasselbalch equation:

  

  pH = + log([conjugate base]/[conjugate acid]) Equation 1

  

  Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, a byproduct of metabolism, and leaves in two forms: exhaled and excreted bicarbonate. Blood pH can be adjusted rapidly by changes

  

  in the rate of exhalation. The reaction given below, which is catalyzed by carbonic anhydrase in the erythrocytes, describes how bicarbonate and interact in the blood.

  

  + + Reaction 1

  How does the titration of a weak monoprotic acid with a strong base differ from the titration of a strong monoprotic acid with a strong base?

  A. The equivalence point will occur at a higher pH.

  B. The equivalence point will occur at a lower pH.

  C. The equivalence point will occur at the same pH.

  D. Whether the equivalence point is higher or lower depends on the particular monoprotic acids used.

  Correct Answer: A

  When a weak acid is reacted with a strong base, the equivalence point will be in the basic region. Consider the titration of equimolar solutions of acetic acid and NaOH. Before the equivalence point, the following reaction takes place:

  

  At the equivalence point, only C2H3O2?exists. When C2H3O2?undergoes hydrolysis (i.e., reacts with water), hydroxide ions are formed according to the following equilibrium:

  

  The numerical value of the equilibrium constant along with the initial concentration of acetate is all that is needed to determine the hydroxide ion concentration. When equimolar solutions of a strong acid and a strong base are titrated, the equivalence point will be neutral. It is neutral because neither of the ions present at the equivalence point can undergo hydrolysis. Choice A is therefore the correct response. Choice B would be correct if a weak base was titrated with a strong acid.

• Question 273:

  It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pH are called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. The table below lists the major buffers found in the blood and/or kidneys. Table 1 Buffer pKa of a typical conjugate acid:*

  

  + Histidine side chains

  

  Organic phosphates N-terminal amino groups

  

  6.1

  6.3

  6.8

  7.0

  8.0

  9.2

  

  *For buffers in many of these categories, there is a range of actual values.

  

  The relationship between blood pH and the of any buffer can be described by the Henderson-Hasselbalch equation: pH = + log([conjugate base]/[conjugate acid]) Equation 1

  

  

  Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, a byproduct of metabolism, and leaves in two forms: exhaled and excreted bicarbonate. Blood pH can be adjusted rapidly by changes

  

  in the rate of exhalation. The reaction given below, which is catalyzed by carbonic anhydrase in the erythrocytes, describes how bicarbonate and interact in the blood.

  

  + + Reaction 1

  The equilibrium as shown in Reaction 1 is most likely to proceed through which of the following intermediates?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: A

  Carbonic acid, H2CO3, is the intermediate formed when carbon dioxide and water combine, making choice A the correct response. You should be able to recognize that when the products of Reaction 1 combine, carbonic acid will result. Choice B is incorrect because it is just not a logical intermediate: water and carbon dioxide would not react to form H+ and CO32-. Choice D and choice C can be eliminated because they both contain carbon dioxide, which is one of the two reactants.

• Question 274:

  It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pH are called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. The table below lists the major buffers found in the blood and/or kidneys. Table 1 Buffer pKa of a typical conjugate acid:*

  

  + Histidine side chains

  

  Organic phosphates N-terminal amino groups

  

  6.1

  6.3

  6.8

  7.0

  8.0

  9.2

  

  *For buffers in many of these categories, there is a range of actual values.

  

  The relationship between blood pH and the of any buffer can be described by the Henderson-Hasselbalch equation:

  

  pH = + log([conjugate base]/[conjugate acid]) Equation 1

  

  Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, a byproduct of metabolism, and leaves in two forms: exhaled and excreted bicarbonate. Blood pH can be adjusted rapidly by changes

  

  in the rate of exhalation. The reaction given below, which is catalyzed by carbonic anhydrase in the erythrocytes, describes how bicarbonate and interact in the blood.

  

  + + Reaction 1

  What would be the order of conjugate acid strength in the following buffers?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  You should know that the smaller the pKa the stronger the acid. Looking at Table 1, it can be seen that the histidine side chains buffer has a stronger conjugate acid than both the organic phosphates buffer and the ammonium buffer. Choice C is therefore the correct response. Choice A is wrong because the pKa of the histidine side chains buffer is not equal to the organic phosphates buffer. Choice B is wrong because it is the reverse of what it should be. Choice D is wrong because, again, the pKa of the histidine side chains buffer is not equal to the organic phosphates buffer.

• Question 275:

  It is critical for the human body blood to maintain its pH at approximately 7.4. Decreased or increased blood pH are called acidosis and alkalosis respectively; both are serious metabolic problems that can cause death. The table below lists the major buffers found in the blood and/or kidneys. Table 1 Buffer pKa of a typical conjugate acid:* + Histidine side chains

  

  

  Organic phosphates N-terminal amino groups

  

  6.1

  6.3

  6.8

  7.0

  8.0

  9.2

  

  *For buffers in many of these categories, there is a range of actual values.

  

  The relationship between blood pH and the of any buffer can be described by the Henderson-Hasselbalch equation:

  

  pH = + log([conjugate base]/[conjugate acid]) Equation 1

  

  Bicarbonate, the most important buffer in the plasma, enters the blood in the form of carbon dioxide, a byproduct of metabolism, and leaves in two forms: exhaled and excreted bicarbonate. Blood pH can be adjusted rapidly by changes

  

  in the rate of exhalation. The reaction given below, which is catalyzed by carbonic anhydrase in the erythrocytes, describes how bicarbonate and interact in the blood.

  

  + + Reaction 1

  If the pH of blood were to increase to 7.6, what would be the likely outcome?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: D

  If the pH of blood increases to 7.6, it becomes more alkaline, and the pH of blood must be kept at approximately pH 7.4. Since it is stated in the second sentence of the last paragraph that blood pH can be adjusted rapidly by changes in the rate of CO2 exhalation, choice A and choice B can be eliminated. In order to bring the pH of blood back to its normal value of 7.4, it must become more acidic; it becomes more acidic by increasing the concentration of H+ Reaction 1 has H+ as a product, and according to Le Chatelier's principle, you should know that a reaction will proceed in a direction that will consume an added reactant or product. In other words, the concentration of a product can be increased by increasing the concentration of a reactant. If the concentration of carbon dioxide is allowed to increase, it will react to produce more H+, resulting in a lowering of the pH. The concentration of carbon dioxide will increase if it is not exhaled, making choice D the correct response.

• Question 276:

  The resistance of a resistor is defined as the ratio of the voltage drop across it to the current passing through it. The resistance of a resistor can be measured using the circuit illustrated in Figure 1.

  

  Figure 1

  In the above circuit, a variable voltage source with negligible internal resistance is connected to a resistor. The voltage across the resistor is measured by a voltmeter and the current through the resistor is measured by an ammeter.

  

  Additional resistors may be added to the circuit. The total resistance can be calculated as follows: If and are two resistances of two resistors, then the total resistance is given by = + when the resistors are connected in

  

  series, and by 1/ = 1/ + 1/ when the resistors are connected in parallel.

  

  Circuits similar to the one above are used in the common household appliance known as the toaster. The rate by which energy in the form of heat is dissipated by the resistor equals , where I is the current that passes through the resistor and R is the resistance of the resistor. Energy is dissipated in a resistor because moving electrons collide with atoms in the resistor, causing the atoms to vibrate.

  As current passes through a resistor, the temperature of the resistor will increase. Which of the following accounts for the temperature increases?

  A. The average kinetic energy of the atoms in the resistor increases as a result of the collisions with the electrons in the current.

  B. The average potential energy of the atoms in the resistor increases as a result of the collisions with the electrons in the current.

  C. The average kinetic energy of the electrons in the current increases as a result of the collisions with the atoms in the resistor.

  D. The average potential energy of the electrons in the current increases as a result of the collisions with the atoms in the resistor.

  Correct Answer: A

  When the electrons collide with the atoms of the resistor, the vibration of the atoms increase, and the atoms' kinetic energy, which is the energy of motion, increases as well. By definition, the temperature of a gas, liquid, or solid is a measure of the average kinetic energy of the atoms that make up the substance. Thus, the increase in the kinetic energy of the atoms is directly related to the increase in temperature, and choice A is correct. Since temperature is related to the average kinetic energy and not the potential energy, choices B and D are wrong. As for choice C, the passage states that the atoms vibrate as a result of the collisions. A moving electron hits an atom which is basically stationary and starts it in motion. This means that the electron transfers some of its kinetic energy to the atom. So the electron's kinetic energy actually decreases as a result of the collisions, and choice C is wrong.

• Question 277:

  The resistance of a resistor is defined as the ratio of the voltage drop across it to the current passing through it. The resistance of a resistor can be measured using the circuit illustrated in Figure 1.

  

  Figure 1

  In the above circuit, a variable voltage source with negligible internal resistance is connected to a resistor. The voltage across the resistor is measured by a voltmeter and the current through the resistor is measured by an ammeter.

  

  Additional resistors may be added to the circuit. The total resistance can be calculated as follows: If and are two resistances of two resistors, then the total resistance is given by = + when the resistors are connected in

  

  series, and by 1/ = 1/ + 1/ when the resistors are connected in parallel.

  

  Circuits similar to the one above are used in the common household appliance known as the toaster. The rate by which energy in the form of heat is dissipated by the resistor equals , where I is the current that passes through the resistor and R is the resistance of the resistor. Energy is dissipated in a resistor because moving electrons collide with atoms in the resistor, causing the atoms to vibrate.

  

  What is the energy delivered to a piece of toast in one second when it is inside a toaster in which a 4 a current passes through a 10-k resistor?

  A. 0.04 J

  B. 0.16 J

  C. 2.5 J

  D. 40 J

  Correct Answer: B

  The energy delivered to a piece of toast must be equal to the energy dissipated by the toaster's resistor. The rate of energy dissipation, or the energy released per unit time, is known as the power. Mathematically, this can be expressed as P = E/t, where P is the power, and E is the energy released in time t. The passage gives the relation P = I2 R, where P is the power, I is the current, and R is the resistance of the resistor. Equating the two expressions for power and solving for E, gives E = I2 Rt. Now we substitute the values given in the question stem into the last equation for the energy and obtain E = (4 x 10-3 A)2(10 x 103)(1 s) = 0.16 J, which is choice B. As is often the case with questions involving calculations, the wrong answer choices are the results of math errors. For example, you would have obtained choice D, if you forgot to square the current.

• Question 278:

  The resistance of a resistor is defined as the ratio of the voltage drop across it to the current passing through it. The resistance of a resistor can be measured using the circuit illustrated in Figure 1.

  

  Figure 1

  In the above circuit, a variable voltage source with negligible internal resistance is connected to a resistor. The voltage across the resistor is measured by a voltmeter and the current through the resistor is measured by an ammeter.

  

  Additional resistors may be added to the circuit. The total resistance can be calculated as follows: If and are two resistances of two resistors, then the total resistance is given by = + when the resistors are connected in

  

  series, and by 1/ = 1/ + 1/ when the resistors are connected in parallel.

  

  Circuits similar to the one above are used in the common household appliance known as the toaster. The rate by which energy in the form of heat is dissipated by the resistor equals , where I is the current that passes through the resistor and R is the resistance of the resistor. Energy is dissipated in a resistor because moving electrons collide with atoms in the resistor, causing the atoms to vibrate.

  In order for the ammeter to have a very small effect on the current flowing through the resistor, the ammeter should:

  A. be connected to the resistor with insulated wire.

  B. be connected as close as possible to the positive terminal of the voltage source.

  C. have a very low resistance.

  D. be sensitive to currents flowing in either direction around the circuit.

  Correct Answer: C

  Ammeters, like all components of electric circuits, have some internal resistance. Therefore, adding an ammeter to the circuit is like adding another resistor in series. If the ammeter has a large resistance, then the current flowing through the resistor will be significantly reduced when it is added to the circuit. If it has a small resistance, the current will be only slightly affected; so choice C is correct. Choice A is wrong. Insulated wires are wires surrounded by a nonconducting material, which prevents unintended contact with the current. Insulating the wires won't directly affect the current through the resistor and won't affect the way that the ammeter works. Choice B is wrong as well. The current in the wire to the right of the resistor is the same as the current to the left of the resistor, so an ammeter will function identically at either location. In both cases, the ammeter is in series with the resistor. Choice D is also wrong. An ammeter that can detect current in either direction might be useful. But whether it can or not is unrelated to the ammeter's effect on the current flowing through the resistor.

• Question 279:

  The resistance of a resistor is defined as the ratio of the voltage drop across it to the current passing through it. The resistance of a resistor can be measured using the circuit illustrated in Figure 1.

  

  Figure 1

  In the above circuit, a variable voltage source with negligible internal resistance is connected to a resistor. The voltage across the resistor is measured by a voltmeter and the current through the resistor is measured by an ammeter.

  

  Additional resistors may be added to the circuit. The total resistance can be calculated as follows: If and are two resistances of two resistors, then the total resistance is given by = + when the resistors are connected in

  

  series, and by 1/ = 1/ + 1/ when the resistors are connected in parallel.

  

  Circuits similar to the one above are used in the common household appliance known as the toaster. The rate by which energy in the form of heat is dissipated by the resistor equals , where I is the current that passes through the resistor and R is the resistance of the resistor. Energy is dissipated in a resistor because moving electrons collide with atoms in the resistor, causing the atoms to vibrate.

  In which direction do the electrons travel, and in which direction does the current flow, in the circuit in Figure 1?

  A. The electrons travel clockwise, and the current flows counterclockwise.

  B. The electrons travel clockwise, and the current flows clockwise.

  C. The electrons travel counterclockwise, and the current flows clockwise.

  D. The electrons travel counterclockwise, and the current flows counterclockwise.

  Correct Answer: C

  Like charges repel each other and opposite charges attract each other. Since electrons have negative charge, they will move away from a negative terminal and move towards a positive terminal. This corresponds to the counterclockwise

  direction. So choices A and B can be ruled out.

  By convention current flows in the direction that positive charge would flow. This means that in the circuit in Figure 1, the current flows in the clockwise direction, from the positive terminal to the negative terminal, and choice C is the correct

  answer. If you couldn't figure out the direction of current flow or the direction that the electrons travel, but you knew they were opposite directions, you could have eliminated choices B and D.

• Question 280:

  The lead-acid battery, also called a lead storage battery, is the battery of choice for starting automobiles. It contains 6 cells connected in series, each composed of a lead oxide cathode "sandwiched" between 2 lead anodes. Insulating separators are placed between the electrodes to prevent internal short-circuits. Aqueous sulfuric acid is the electrolyte.

  When the battery is being discharged, the following reaction takes place:

  

  Reaction 1

  The electrode reactions, both written as reductions, are shown in Table 1.

  Table 1

  Half-reaction

  E?V)

  PbO2(s) + SO42-(aq) + 4H+(aq) + 2ePbSO4(s) + 2H2O PbSO4(s) + 2e-Pb(s) + SO42-(aq)

  ?.36

  As a car operates, the battery is recharged by electricity produced by the car's alternator, an AC generator whose ultimate power source is the car's internal combustion engine. In spite of this, batteries eventually lose their power. The battery is said to be "dead" when Reaction 1 has proceeded completely to the right.

  Currents as small as 0.1 A can be fatal to humans. If the typical resistance of the human body is 10 k, what is the minimum voltage that could be fatal?

  A. 0.1 V

  B. 1 V

  C. 100 V

  D. 1,000 V

  Correct Answer: D

  Remembering that a k is 103 and using the relationship that V = IR, a current of 0.1 A times a resistance of 10,000 gives a voltage of 1000 V. Choice D is therefore the correct answer. Choice B would have been chosen if you did not convert kiloohms to ohms. Choice A or choice D could have been chosen if you made other conversion errors.