Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 181:

  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

  In the body, rapid breathing (hyperventilation) leads to a decrease in the concentration of carbon dioxide in the blood. This will tend to:

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  Removing CO2 would shift the equilibrium shown below towards the left, decreasing the H+ concentration in the blood and thus, increasing the pH. The body uses this technique (hyperventilation) as a means of regulating plasma pH.

  

• Question 182:

  

  Given the following H?values, calculate the H?of formation of (s).

  

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The H?for a reaction can be calculated as the difference between the heat of formation of the products and that of the reactants.

  

  Recall that heat of formation is the heat of the reaction that produces 1 mol of the material in question from pure elements in their standard states. The heat of formation of elements in their standard states is zero. The passage provides the

  

  reaction for formation of

  

  So we can determine the heat of formation of from the first equation and the of given by the equation above.

  

  You should, of course, use approximations judiciously to save time.

• Question 183:

  Ink jet printers produce high resolution output, at a lower cost than laser printers, by generating charged ink droplets which are then deflected onto a sheet of paper by an electric field. Each droplet deflected by the field strikes the paper and forms a tiny dot of ink. While a typical printed letter requires about 100 drops, an ink jet printer is able to produce drops at a rate of 100,000 per second.

  

  The essential elements of the ink jet printer head are shown in Figure 1. The drop generator produces the ink droplets, each with a mass of approximately 1.2 kg and a diameter of approximately 30 m. The drops then enter a highly precise charging unit which controls the charge q on each droplet to within 2%, with typical charges for drops generated by various ink jet printers ranging from ?.0 C to ?.0 C. The charged droplets are

  

  

  subsequently passed through the deflecting plates between which a variable electric field is generated. The electronically controlled electric field between the plates is typically varied over a range from 1.0 N/C to 5.4 N/C,

  

  and is used to aim the ink droplet at the paper. .

  In order to observe the velocity of the ink droplet, a magnetic field is generated to exactly counter the force applied to the ink droplet by the electric field. This magnetic field is oriented:

  A. towards the left.

  B. towards the right.

  C. out of the page.

  D. into the page.

  Correct Answer: D

  The negatively charged particle is accelerated upwards by the downwards directed electric field. Therefore, the magnetic field must apply a downwards force to counter the upwards electric force. According to the right hand rule, a positive particle travelling towards the left must move through a magnetic field directed into the page to experience a downwards force. This is equivalent to a negative charge moving towards the right through a magnetic field directed into the page.Choices A and B are incorrect because a magnetic field directed towards the right would apply no force to a charged particle moving to the right (because the two vectors are parallel). Choice C is incorrect because a magnetic field directed out of the page would apply an upwards force on a negative particle travelling towards the right. This is the same direction as the force applied by the electric field.

• Question 184:

  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

  

  is completely insoluble in aqueous solution. Addition of barium chloride to a phosphoric acid solution will decrease the pH because:

  A. barium acts as a Lewis base.

  B. barium chloride reduces the concentration of hydrogen ions in solution.

  C. chloride ion acts to stabilize the hydrogen ions in solution.

  D. the precipitation of barium phosphate drives the acid dissociation to completion.

  Correct Answer: D

  The passage states that barium phosphate is insoluble and will precipitate. This will drive the reaction shown below to completion:

  

  The [H+] will increase and the pH will drop. Choice A is incorrect because a Lewis base is an electron pair donor. Ba2+is not an electron pair donor. Choice B is incorrect because a reduction in the hydrogen ion concentration would lead to an

  increase in pH.

  Choice C is incorrect because the chloride ion is a spectator in this reaction and does not participate in the formation of barium phosphate precipitate.

• Question 185:

  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

  Which region of the graph in Figure 1 provides the best buffering around neutral pH?

  A. A

  B. B

  C. C

  D. D

  Correct Answer: C

  The best buffering occurs in the flat regions of the titration curve where the concentrations of acid and conjugate base are equal. Points A and C are good buffering regions, but point C is the best buffering region around neutral (7) pH.

• Question 186:

  Ink jet printers produce high resolution output, at a lower cost than laser printers, by generating charged ink droplets which are then deflected onto a sheet of paper by an electric field. Each droplet deflected by the field strikes the paper and forms a tiny dot of ink. While a typical printed letter requires about 100 drops, an ink jet printer is able to produce drops at a rate of 100,000 per second.

  

  The essential elements of the ink jet printer head are shown in Figure 1. The drop generator produces the ink droplets, each with a mass of approximately 1.2 kg and a diameter of approximately 30 m. The drops then enter a

  

  highly precise charging unit which controls the charge q on each droplet to within 2%, with typical charges for drops generated by various ink jet printers ranging from ?.0 C to ?.0 C. The charged droplets are

  

  subsequently passed through the deflecting plates between which a variable electric field is generated. The electronically controlled electric field between the plates is typically varied over a range from 1.0 N/C to 5.4 N/C,

  

  and is used to aim the ink droplet at the paper. .

  

  An ink jet printer deflects a particular ink droplet by 1.5 mm in the region of the deflector. Which of the following is a possible value of the work done on the droplet?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  To deflect an ink droplet, the deflector must do work on the drop. The formula for calculating work is . To answer the question we must calculate the force generated by the electric field of the deflector. Because the electric field is uniform, the electric force will also be uniform. The electric force is related to electric field by the formula F = Eq.So, to calculate the work done by the drop, we can use the formula W = E X q X d. (The field and the displacement are parallel, so = 0?and cos = 1.) The question gives the distance as 1.5 mm and the passage gives a range of values for the field generated by the deflector and for the charge on the oil droplet. Because the answer choices are fairly spread out, we can calculate using average values for the drop charge and field strength.

  

  

  Choice A is incorrect because work is done by the electric field. If a magnetic field were involved, no work would be done because the force is always perpendicular to the direction of movement.

• Question 187:

  Ink jet printers produce high resolution output, at a lower cost than laser printers, by generating charged ink droplets which are then deflected onto a sheet of paper by an electric field. Each droplet deflected by the field strikes the paper and forms a tiny dot of ink. While a typical printed letter requires about 100 drops, an ink jet printer is able to produce drops at a rate of 100,000 per second.

  

  The essential elements of the ink jet printer head are shown in Figure 1. The drop generator produces the ink droplets, each with a mass of approximately 1.2 kg and a diameter of approximately 30 m. The drops then enter a

  

  highly precise charging unit which controls the charge q on each droplet to within 2%, with typical charges for drops generated by various ink jet printers ranging from ?.0 C to ?.0 C. The charged droplets are

  

  subsequently passed through the deflecting plates between which a variable electric field is generated. The electronically controlled electric field between the plates is typically varied over a range from 1.0 N/C to 5.4 N/C,

  

  and is used to aim the ink droplet at the paper. .

  Suppose that an ink-jet printer head is programmed to produce the letter shown below (note that cross hairs are not produced by the ink jet printer head).

  

  An uniform external electric field pointing upwards is superimposed on the printer head, and the letter is printed again. Which of the following diagrams best represents the new printer output?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: B

  Superimposing a uniform electric field pointing upwards would lead to a uniform decrease in the electric field each ink droplet experiences. A uniform decrease in the electric field will decrease the acceleration of each of the droplets used to produce the letter. Thus, the letter will be shifted uniformly. Only choice B shows a uniform shift of the letters. Recall that the force on a particle in an electric field is given by F = Eq. If the field (E) is increased and the charge does not change, then the force will increase.Choice A is incorrect because it is identical to the original letter. Since the droplets are deflected by an electric field, the entire pattern would be shifted by a uniform decrease in the electric field.Choice C is incorrect because it shows a pattern in which the letter is reduced in size. This pattern might arise if the field decrease were not uniform. For example, if the field were decreased proportionally (decreased by a certain percentage), we would expect a scaling of the letter to occur.Choice D is incorrect because it shows rotation of the letter. This would not occur in a uniform electric field.

• Question 188:

  

  In the circuit below, = = 10 ohms. Removing will have which of the following effects on the current through ?

  

  A. Decrease to 1/2

  B. No change

  C. Double

  D. Increase by factor of 10

  Correct Answer: B

  The two resistors are wired in parallel, and so the voltage across each is the same. In this particular case, since there are no other resistive elements in the circuit, the voltage across each is the same as the emf of the battery, V. The current

  through R1 and R2 is therefore V/ R1 and V/ R2 respectively. Removing R1 will not change the voltage across the parallel resistor R2: From Kirchhoff's law, the voltage across R2 is still V, and so there will be no change in the current through

  R2.

  (Note, however, the current through the battery will decrease (to 1/2 its original value).)

• Question 189:

  Ink jet printers produce high resolution output, at a lower cost than laser printers, by generating charged ink droplets which are then deflected onto a sheet of paper by an electric field. Each droplet deflected by the field strikes the paper and forms a tiny dot of ink. While a typical printed letter requires about 100 drops, an ink jet printer is able to produce drops at a rate of 100,000 per second.

  

  The essential elements of the ink jet printer head are shown in Figure 1. The drop generator produces the ink droplets, each with a mass of approximately 1.2 kg and a diameter of approximately 30 m. The drops then enter a

  

  highly precise charging unit which controls the charge q on each droplet to within 2%, with typical charges for drops generated by various ink jet printers ranging from ?.0 C to ?.0 C. The charged droplets are subsequently passed through the deflecting plates between which a variable electric field is generated. The electronically controlled electric field between the plates is typically varied over a range from 1.0 N/C to 5.4 N/C,

  

  

  and is used to aim the ink droplet at the paper. .

  

  In which direction would the deflector in Figure 1 deflect the ink drop?

  A. Upwards in the plane of the page

  B. Downwards in the plane of the page

  C. Into the plane of the page

  D. Out of the plane of the page

  E. (0,1) and (2,1)

  F. (0,1) and (0,2)

  G. (1,1) and (3,1)

  H. (2,1) and (1,2)

  Correct Answer: A

  An electric field indicates the direction in which a positive charge would be accelerated. The ink droplet is negatively charged and will thus be accelerated in the opposite direction to that of the electric field. Since the electric field in Figure 1 points down, the charged ink droplet will be defelected upwards in the plane of the page. Choice B is incorrect because a positive charge would be deflected downwards in the plane of the page. Choices C and D are incorrect applications of the right hand rule. The right hand rule is used to determine the force on a charge moving through a magnetic field. This problem concerns movement of a charge through an electric field.

• Question 190:

  A 2-stage rocket is launched vertically upwards at velocity v. At height h it discards its first stage in four fragments and travels faster without changing direction. Which of the following views from above could be the velocity vectors of the first stage fragments?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: D

  The momentum of the rocket prior to discarding its first stage will equal the momentum of the rocket (and its parts) following loss of the first stage. Its initial momentum is purely vertical (in the Z-direction), and so, the X and Y components of the momentum following loss of the first stage must sum to zero. The only choice that allows the vector sum of these horizontal momen-tums to cancel out is choice D. Note that the discarded masses do not need to be equal, given the answer choices.