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Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 19 )

Updated On: 30-Jan-2026
Page 19 of 164
PDF with 811 Questions

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?

  1. -3
  2. 0
  3. +3
  4. +6

Answer(s): C

Explanation:

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.



A researcher in a molecular biology lab planned to carry out an extraction procedure known as an alkaline plasmid prep, which is designed to purify plasmids, small pieces of the hereditary material DNA, from bacterial cells. The bacteria are first placed into a test tube containing liquid nutrient medium and allowed to grow until they reach a high population density. The culture, which consists of solid cells suspended in the medium, is then centrifuged; a solid pellet is formed. The supernatant is poured out, leaving the pellet behind, and the cells are resuspended in a mL of lysis buffer solution (50 mM glucose, 25 mM Tris buffer and 10 mM ethylenediaminetetraacetic acid (EDTA), with 5 mg of the enzyme lysozyme added). They are then incubated for 30 minutes at 0° C, during which time the bacterial cell walls break down and the cell contents are released into the solution. After incubation, 1 mL of 0.4 N sodium hydroxide and 1 mL of 2% sodium dodecyl sulfate (SDS) are added, and the solution is again incubated on ice for 10 minutes. 2 mL of 3 M sodium acetate are added and the mixture is incubated for 30 minutes at 0° C. The test tube is centrifuged once more and the supernatant is decanted into a clean tube, leaving behind the protein and most other cell components in the pellet.
Finally, 10 mL of pure ethanol are added to the supernatant from the previous step to precipitate out the DNA, and the test tube is incubated at -20° C for 60 minutes, during which the mixture remains liquid. The mixture is centrifuged a final time and the supernatant removed. The translucent precipitate that results is washed with 70% ethanol (70% ethanol and 30% water by volume), allowed to dry, and resuspended in 1 mL of TE buffer (10 mM Tris, 1 mM EDTA).
In preparation for this experiment, the researcher prepared stock solutions of the various chemicals that she will need in the experiment. Stock solutions are highly concentrated solutions of commonly used chemicals in water from which dilute solutions are prepared for daily use. Table 1 shows the chemicals, their molecular formulas and weights, and the composition of commonly used stock solutions.

Which of the following conclusions can be reached based on the fact that DNA precipitates in the last step of the plasmid prep procedure?

  1. DNA dissolves better in water at lower temperatures.
  2. DNA is polar and therefore dissolves better in water than in a mixture of water and ethanol.
  3. DNA is nonpolar and therefore dissolves better in ethanol than in water.
  4. DNA dissolves well in ethanol and precipitates only because the solution is centrifuged.

Answer(s): B

Explanation:

In the last step of the plasmid prep procedure, ethanol is added to the mixture, which is an aqueous solution, or a solution whose solvent is water. Then the test tube is incubated at low temperature for an hour, and finally the solution is centrifuged and a DNA precipitate forms. This happens because DNA, or deoxyribonucleic acid, is a highly polar substance, and is therefore more soluble in an aqueous solution than in a solvent composed mostly of ethanol. Thus, choice B is correct. Remember the rule that like dissolves like: thus a highly polar substance will dissolve better in a more polar solvent ­ water ­ than in a less polar solvent ­ ethanol. Choice c is wrong because there's no evidence in the passage that DNA dissolves better in ethanol than in water; in fact, there's evidence against this conclusion, since the DNA is fully dissolved in the water solution but precipitates out of the water-ethanol solution. Choice D is wrong because centrifugation can't make a substance precipitate out of a solution; it can only make a precipitate that is suspended in the solution settle to the bottom. Finally, choice A is wrong because there's no evidence that DNA dissolves better at lower temperatures; on the contrary, the incubation at -20° apparently contributes to its precipitation.



What is the shape of a molecule of NH3?

  1. Trigonal planar
  2. Pyramidal
  3. Tetrahedral
  4. Trigonal bipyramidal

Answer(s): B

Explanation:

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.



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?

  1. 66 cm
  2. 68 cm
  3. 70 cm
  4. 74 cm

Answer(s): B

Explanation:

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.



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

  1. Doubling the distance between the particles
  2. Halving the charge on each particle
  3. Halving the charge on only one of the particles
  4. Placing a positively charged particle midway between the particles

Answer(s): C

Explanation:

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.



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