Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 241:

  If a spring is 64 cm long when it is unstretched and is 8% longer when a 0.5-kg mass hangs from it, how long will it be with a 0.4-kg mass suspended from it?

  A. 66 cm

  B. 68 cm

  C. 70 cm

  D. 74 cm

  Correct Answer: B

  To solve this problem, apply Hooke's law F = kx, where F is the force applied to the spring, x is the distance the spring stretches, and k is the spring constant. The forces applied in this case are weights. Since weight is proportional to mass, the distance the spring stretches is also proportional to the mass of the object attached to the spring.

  The spring starts out with a length of 64 cm. When the 0.5 kg mass is attached, it stretches 8% longer, or 64(0.08) = 5.12 cm. Since the distance stretched and the mass attached are proportional, the ratio of the distance stretched to the mass attached is (5.12 cm)/(0.5 kg) 5/(0.5) = 10 cm/kg. In other words, each kilogram attached to the spring stretches it 10 cm. Thus, when the 0.4 kg mass is attached, the spring is stretched a distance (0.4 kg)(10 cm/kg) = 4 cm. Therefore, the total length of the spring is its original length plus the distance it stretches, or 64 + 4 = 68 cm. This is choice B.

• Question 242:

  Which of the following will halve the magnitude of the electrostatic force of attraction between two charged particles?

  A. Doubling the distance between the particles

  B. Halving the charge on each particle

  C. Halving the charge on only one of the particles

  D. Placing a positively charged particle midway between the particles

  Correct Answer: C

  The electrostatic force between two charged particles is given by F = kq1kq2/r2, where k is a constant, kq1 and kq2 are the charges on the particles, and r is the distance between the two charges. Since the force is proportional to each of the charges, halving the charge on one of the particles will also halve the force, and choice C is correct. Halving the charge on both particles will reduce the force by one fourth, so choice B is wrong. Since the force is inversely proportional to the square of the distance, doubling the distance will reduce the force by one fourth, and choice A is wrong. Watch out for choice D. It is wrong because the presence of a third charge will not change the force of attraction between the first and second charges. The third charge, however, will change the net force on the first and second charges because it interacts with each separately.

• Question 243:

  Arsenic is widely distributed in sulfide ores of many metals and is obtained as a byproduct of copper smelting. The element, as well as many compounds of arsenic -- for example arsine, AsH3 -- are extremely poisonous. Arsenic compounds, as might be expected, have found use in herbicides and pesticides, but have also been successful in some pharmacological agents. The first useful antisyphilitic agent, Salvarsan, or 3,3'-diamino-4,4'-dihydroxyarsenobenzene dihydrochloride, is an arsenic compound. The element sublimes at 600°C, forming tetrahedral molecules, As4. Arsenic is a metalloid, possessing properties characteristic of both metals and non-metals. Arsenic is a gray-colored, metalliclooking solid, but arsenic vapor is yellow in color, has a garlic-like odor, and is very poisonous. If the arsenic vapor is cooled rapidly, an unstable, yellow crystalline allotrope consisting of As4 molecules is produced. The Marsh test, based on the instability of arsine, is a very sensitive test for the presence of arsenic. This test is commonly employed in the detection of arsenic poisoning -- either before or after death. The apparatus for the Marsh test is shown in Figure 1.

  

  Figure 1

  Typically, a sample, usually hair, is taken from a person suspected of being the victim of arsenic poisoning. This sample is then treated in such a way so as to produce arsenic oxide, As4O6. The oxide is then placed into the apparatus shown

  in Figure 1 and reacted according to Reaction 1.

  As4O6 + 12Zn(s) + 24H+(aq) AsH3(g) + 12Zn2 +(aq) + 6H2O

  Reaction 1

  When the evolved arsine is ignited it decomposes into its elements. The arsenic vapor is rapidly cooled when it encounters the porcelain evaporating dish and deposits a black mirror of arsenic on the bottom, indicating the presence of arsenic

  in the original sample.

  A common ore of arsenic is called orpiment, As2S3. What is the oxidation state of arsenic in orpiment?

  A. ?

  B. 0

  C. +3

  D. +6

  Correct Answer: C

  Because there are three sulfides in orpiment and the molecule has no charge overall, the total negative charge of the sulfides, minus six, must be balanced by the positive charges of the arsenics. The two arsenics, therefore, have a collective charge of plus six, and each arsenic has a charge of plus three. Choice C is the correct response.

• Question 244:

  What is the shape of a molecule of NH3?

  A. Trigonal planar

  B. Pyramidal

  C. Tetrahedral

  D. Trigonal bipyramidal

  Correct Answer: B

  Ammonia has a central nitrogen atom, bonded to three hydrogen atoms. According to the valence-shell electron-pair repulsion theory, or VSEPR theory, a central atom surrounded by three bonded pairs of electrons and one lone pair of electrons has a pyramidal geometry. Choice A is wrong because a trigonal planar molecule has the central atom surrounded by three bonding pairs of electrons and no lone pairs, such as BF3. Choice C is wrong because a tetrahedral molecule has the central atom surrounded by four bonding pairs of electrons and no lone pairs, such as CH4. Choice D is wrong because a trigonal bipyramidal shape has the central atom surrounded by five bonding electron pairs and no lone pairs, such as PFs.

• Question 245:

  Arsenic is widely distributed in sulfide ores of many metals and is obtained as a byproduct of copper smelting. The element, as well as many compounds of arsenic -- for example arsine, AsH3 -- are extremely poisonous. Arsenic compounds, as might be expected, have found use in herbicides and pesticides, but have also been successful in some pharmacological agents. The first useful antisyphilitic agent, Salvarsan, or 3,3'-diamino-4,4'-dihydroxyarsenobenzene dihydrochloride, is an arsenic compound. The element sublimes at 600°C, forming tetrahedral molecules, As4. Arsenic is a metalloid, possessing properties characteristic of both metals and non-metals. Arsenic is a gray-colored, metalliclooking solid, but arsenic vapor is yellow in color, has a garlic-like odor, and is very poisonous. If the arsenic vapor is cooled rapidly, an unstable, yellow crystalline allotrope consisting of As4 molecules is produced. The Marsh test, based on the instability of arsine, is a very sensitive test for the presence of arsenic. This test is commonly employed in the detection of arsenic poisoning -- either before or after death. The apparatus for the Marsh test is shown in Figure 1.

  

  Figure 1

  Typically, a sample, usually hair, is taken from a person suspected of being the victim of arsenic poisoning. This sample is then treated in such a way so as to produce arsenic oxide, As4O6. The oxide is then placed into the apparatus shown in Figure 1 and reacted according to Reaction 1. As4O6 + 12Zn(s) + 24H+(aq) AsH3(g) + 12Zn2 +(aq) + 6H2O Reaction 1 When the evolved arsine is ignited it decomposes into its elements. The arsenic vapor is rapidly cooled when it encounters the porcelain evaporating dish and deposits a black mirror of arsenic on the bottom, indicating the presence of arsenic in the original sample.

  If equal masses of gray arsenic and yellow arsenic are allowed to completely react with oxygen at 298 K and constant pressure to form As4O6, which would produce more heat and why?

  A. The yellow, because it is less stable than the gray.

  B. The gray, because it is more stable than the yellow.

  C. Both would produce the same amount of heat because they form the same product.

  D. Both would produce the same amount of heat because they are the same element.

  Correct Answer: A

  It is stated in the last sentence of the first paragraph that yellow arsenic is unstable. Since it is less stable than gray arsenic, meaning that it is at a higher potential energy, more energy will be released when it converts to As4O6. Choice A is the correct response. Choice B is wrong because gray arsenic, being more stable than the yellow, will release less energy. Choice C and choice D are wrong because, as was just discussed, they release different amounts of energy.

• Question 246:

  Arsenic is widely distributed in sulfide ores of many metals and is obtained as a byproduct of copper smelting. The element, as well as many compounds of arsenic -- for example arsine, AsH3 -- are extremely poisonous. Arsenic compounds, as might be expected, have found use in herbicides and pesticides, but have also been successful in some pharmacological agents. The first useful antisyphilitic agent, Salvarsan, or 3,3'-diamino-4,4'-dihydroxyarsenobenzene dihydrochloride, is an arsenic compound. The element sublimes at 600°C, forming tetrahedral molecules, As4. Arsenic is a metalloid, possessing properties characteristic of both metals and non-metals. Arsenic is a gray-colored, metalliclooking solid, but arsenic vapor is yellow in color, has a garlic-like odor, and is very poisonous. If the arsenic vapor is cooled rapidly, an unstable, yellow crystalline allotrope consisting of As4 molecules is produced. The Marsh test, based on the instability of arsine, is a very sensitive test for the presence of arsenic. This test is commonly employed in the detection of arsenic poisoning -- either before or after death. The apparatus for the Marsh test is shown in Figure 1.

  

  Figure 1

  Typically, a sample, usually hair, is taken from a person suspected of being the victim of arsenic poisoning. This sample is then treated in such a way so as to produce arsenic oxide, As4O6. The oxide is then placed into the apparatus shown

  in Figure 1 and reacted according to Reaction 1.

  As4O6 + 12Zn(s) + 24H+(aq) AsH3(g) + 12Zn2 +(aq) + 6H2O

  Reaction 1

  When the evolved arsine is ignited it decomposes into its elements. The arsenic vapor is rapidly cooled when it encounters the porcelain evaporating dish and deposits a black mirror of arsenic on the bottom, indicating the presence of arsenic

  in the original sample.

  The Marsh test takes advantage of the fact that arsine is not very soluble in water. Since arsenic is below nitrogen on the periodic table, it would be expected that arsine, like ammonia, would be very soluble in water. What is the most likely

  reason for this difference in solubility?

  A. Arsine has a higher molecular weight than ammonia does.

  B. Arsine has a smaller dipole moment than ammonia does.

  C. Arsine is less basic than ammonia is.

  D. Arsine is less stable than ammonia is.

  Correct Answer: B

  The solubility of one compound in another is usually governed by the rule that "like dissolves like." Water and ammonia, both being polar, are quite soluble in each other. Arsine has a similar geometry to that of ammonia, but the electronegativity of arsenic is substantially less than that of nitrogen, and remember: ammonia can hydrogen bond with water, arsine cannot. In fact, the electronegativity of arsenic is very close to that of hydrogen. Because of arsenic's electronegativity, arsine has a small dipole moment, rendering it insoluble in polar solvents such as water. Choice B is the correct response. While choice A, choice C, and choice D are true statements, each are less important factors in influencing the solubility of compounds, especially in the case of arsine.

• Question 247:

  Arsenic is widely distributed in sulfide ores of many metals and is obtained as a byproduct of copper smelting. The element, as well as many compounds of arsenic -- for example arsine, AsH3 -- are extremely poisonous. Arsenic compounds, as might be expected, have found use in herbicides and pesticides, but have also been successful in some pharmacological agents. The first useful antisyphilitic agent, Salvarsan, or 3,3'-diamino-4,4'-dihydroxyarsenobenzene dihydrochloride, is an arsenic compound. The element sublimes at 600°C, forming tetrahedral molecules, As4. Arsenic is a metalloid, possessing properties characteristic of both metals and non-metals. Arsenic is a gray-colored, metalliclooking solid, but arsenic vapor is yellow in color, has a garlic-like odor, and is very poisonous. If the arsenic vapor is cooled rapidly, an unstable, yellow crystalline allotrope consisting of As4 molecules is produced. The Marsh test, based on the instability of arsine, is a very sensitive test for the presence of arsenic. This test is commonly employed in the detection of arsenic poisoning--either before or after death. The apparatus for the Marsh test is shown in Figure 1.

  

  Figure 1

  Typically, a sample, usually hair, is taken from a person suspected of being the victim of arsenic poisoning. This sample is then treated in such a way so as to produce arsenic oxide, As4O6. The oxide is then placed into the apparatus shown

  in Figure 1 and reacted according to Reaction 1.

  As4O6 + 12Zn(s) + 24H+(aq) AsH3(g) + 12Zn2 +(aq) + 6H2O

  Reaction 1

  When the evolved arsine is ignited it decomposes into its elements. The arsenic vapor is rapidly cooled when it encounters the porcelain evaporating dish and deposits a black mirror of arsenic on the bottom, indicating the presence of arsenic

  in the original sample.

  The phase diagram for arsenic is shown below. At what point does liquid arsenic exist?

  

  A. 1.0 bar and 874 K

  B. 1.0 bar and 1673 K

  C. 223 bar and 1672 K

  D. 223 bar and 873 K

  Correct Answer: C

  Explanation: This question is testing your knowledge of phase diagrams. You should know that Region I -- the lower temperature region -- is where the solid state exists, Region II -- the higher temperature and pressure region -- is where the liquid phase exists, and that Region III -- the higher temperature and lower pressure region -- is where the vapor phase exists. Choice C is correct because at this temperature and pressure the liquid phase exists. Choice A and choice B are wrong because the vapor phase exists at these points. Choice D is wrong because the solid phases exist at this point.

• Question 248:

  Arsenic is widely distributed in sulfide ores of many metals and is obtained as a byproduct of copper smelting. The element, as well as many compounds of arsenic -- for example arsine, AsH3 -- are extremely poisonous. Arsenic compounds, as might be expected, have found use in herbicides and pesticides, but have also been successful in some pharmacological agents. The first useful antisyphilitic agent, Salvarsan, or 3,3'-diamino-4,4'-dihydroxyarsenobenzene dihydrochloride, is an arsenic compound. The element sublimes at 600°C, forming tetrahedral molecules, As4. Arsenic is a metalloid, possessing properties characteristic of both metals and non-metals. Arsenic is a gray-colored, metalliclooking solid, but arsenic vapor is yellow in color, has a garlic-like odor, and is very poisonous. If the arsenic vapor is cooled rapidly, an unstable, yellow crystalline allotrope consisting of As4 molecules is produced. The Marsh test, based on the instability of arsine, is a very sensitive test for the presence of arsenic. This test is commonly employed in the detection of arsenic poisoning--either before or after death. The apparatus for the Marsh test is shown in Figure 1.

  

  Figure 1 Typically, a sample, usually hair, is taken from a person suspected of being the victim of arsenic poisoning. This sample is then treated in such a way so as to produce arsenic oxide, As4O6. The oxide is then placed into the apparatus shown in Figure 1 and reacted according to Reaction 1. As4O6 + 12Zn(s) + 24H+(aq) AsH3(g) + 12Zn2 +(aq) + 6H2O Reaction 1

  When the evolved arsine is ignited it decomposes into its elements. The arsenic vapor is rapidly cooled when it encounters the porcelain evaporating dish and deposits a black mirror of arsenic on the bottom, indicating the presence of arsenic in the original sample.

  What is the most likely purpose of the calcium chloride in Figure 1?

  A. To remove water from the evolved arsine gas

  B. To remove HCl from the evolved arsine gas

  C. To react with the zinc ion, making the reaction go to completion

  D. To react with the evolved arsine gas

  Correct Answer: A

  Calcium chloride, by forming solid hydrates with water, is often used as a desiccant, or drying agent. It is used in this experiment because the presence of water might prevent the arsine gas from igniting. Choice A is therefore the correct response. Choice B is wrong because calcium chloride is not basic, rendering it ineffective in removing HCl. Choice C is wrong because -- as seen in Figure 1 -- calcium chloride and zinc ion do not have any contact. Choice D is wrong because arsine gas is the analyte, removing it would defeat the purpose of the test.

• Question 249:

  One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus. 14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant. A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.

  In generating 14C in the upper atmosphere, a 14C nucleus combines with a neutron to form a 14C nucleus and:

  A. a proton.

  B. an electron.

  C. a 4He nucleus.

  D. a neutron.

  Correct Answer: A

  

  To answer this question, you have to balance a nuclear reaction. The question stem suggests the reaction , which has a nitrogen nucleus and a neutron on the left side and a carbon nucleus and the unknown particle on the right.

  Two things must be balanced in a nuclear reaction: the charge of the nucleus (which corresponds to the number of protons), and the number of nucleons (which is the number of protons plus the number of neutrons). Balancing nuclear charge, we obtain 7 + 0 = 6 + b, which implies b = 1. Balancing the number of nucleons, we obtain 14 + 1 = 14 + a, which implies a = 1. Thus, the unknown particle is a nucleon with a charge of +1. The only particle that fits both criteria is the proton, choice A. Choice B is wrong because an electron is not a nucleon and it has a charge of ?. Choice C is wrong because a helium nucleus has 4 nucleons and a charge of +2. Choice D is wrong because a neutron has a charge of 0.

• Question 250:

  One of the most common methods that scientists use to determine the age of fossils is known as carbon dating. 14C is an unstable isotope of carbon that undergoes beta decay with a half-life of approximately 5,730 years. Beta decay occurs when a neutron in the nucleus decays to form a proton and an electron which is ejected from the nucleus. 14C is generated in the upper atmosphere when 14N, the most common isotope of nitrogen, is bombarded by neutrons. This mechanism yields a global production rate of 7.5 kg per year of 14C, which combines with oxygen in the atmosphere to produce carbon dioxide. Both the production and the decay of 14C occur simultaneously. This process continues for many half-lives of 14C, until the total amount of 14C approaches a constant. A fixed fraction of the carbon ingested by all living organisms will be 14C. Therefore, as long as an organism is alive, the ratio of 14C to 12C that it contains is constant. After the organism dies, no new 14C is ingested, and the amount of 14C contained in the organism will decrease by beta decay. The amount of 14C that must have been present in the organism when it died can be calculated from the amount of 12C present in a fossil. By comparing the amount of 14C in the fossil to the calculated amount of 14C that was present in the organism when it died, the age of the fossil can be determined.

  After a 14C nucleus decays, the electron that is emitted enters lead and is stopped. What percentage of its kinetic energy does the electron transfer to lead?

  A. 25%

  B. 33%

  C. 50%

  D. 100%

  Correct Answer: D

  Kinetic energy, the energy of motion, is given by K.E. = (1/2)mv2 , where m is the mass of the object and v is its speed. The electron emitted when a 14C nucleus decays is emitted with some initial speed and thus has kinetic energy. After it enters lead and is stopped, the electron's speed is zero so its kinetic energy is also zero. In other words, all the electron's kinetic energy is gone. Since energy is conserved, 100% of the electron's kinetic energy must have been transferred somewhere, and in this case, it is transferred to the lead. Hence, choice D is correct.