Cisco
CompTIA
ISC
EC-Council
EXIN
Fortinet
ITIL
Juniper
Microsoft
VMware
Avaya
SAP
Google
NVIDIA
Salesforce
Amazon
Palo Alto Networks
ISC2
Splunk
ServiceNow
All Vendors
  2. Home
  3. Exams
  4. Test Prep
  5. MCAT Test

Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 18 )

Updated On: 30-Jan-2026
Page 18 of 164
PDF with 811 Questions

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?

  1. 25%
  2. 33%
  3. 50%
  4. 100%

Answer(s): D

Explanation:

Kinetic energy, the energy of motion, is given by KE = (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.



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?

  1. 1.0 bar and 874 K
  2. 1.0 bar and 1673 K
  3. 223 bar and 1672 K
  4. 223 bar and 873 K

Answer(s): 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.



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?

  1. To remove water from the evolved arsine gas
  2. To remove HCl from the evolved arsine gas
  3. To react with the zinc ion, making the reaction go to completion
  4. To react with the evolved arsine gas

Answer(s): A

Explanation:

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.



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?

  1. The yellow, because it is less stable than the gray.
  2. The gray, because it is more stable than the yellow.
  3. Both would produce the same amount of heat because they form the same product.
  4. Both would produce the same amount of heat because they are the same element.

Answer(s): A

Explanation:

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.



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?

  1. Arsine has a higher molecular weight than ammonia does.
  2. Arsine has a smaller dipole moment than ammonia does.
  3. Arsine is less basic than ammonia is.
  4. Arsine is less stable than ammonia is.

Answer(s): B

Explanation:

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.



Viewing page 18 of 164
Viewing questions 86 - 90 out of 811 questions



Post your Comments and Discuss Test Prep MCAT Test exam prep with other Community members:

Join the MCAT Test Discussion