Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 71:

  How many electrons does gold (Au) possess that have a quantum number l = 2?

  B. 23

  C. 29

  D. 79

  Correct Answer: C

  You must know that the quantum number l can take on four different values: 0, 1, 2, 3. Further, you must know that each number corresponds to a particular orbital: 0=s, 1=p, 2=d, 3=f. So, the question is asking us how many d electrons gold possesses. Since it is in the 6th period, it has the following d orbitals: 3d, 4d, 5d. The 3d and 4d orbitals are filled with 10 electrons each, and the 5d orbital has 9 electrons, giving a total of 29. Therefore, the correct answer is C.

• Question 72:

  Given the reduction potentials for the following half-reactions:

  Br2 (l) + 2e? 2 Br?aq) Eored= + 1.07 V

  Au3+ (aq) + 3 e? Au (s) Eored= + 1.55 V

  What is the electrochemical potential for the following chemical reaction?

  2Au3+ (aq) + 6Br?aq) 3Br2 + 2Au (s)

  Eocell = ?

  A. Eo= -0.48 V

  B. Eo= -0.11 V

  C. Eo= +0.11 V

  D. Eo= +0.48 V

  Correct Answer: D

  This question is testing your understanding of electrochemical cells and using half-reactions to determine the cell potential. To answer this question, you must first know that the electrochemical potential for a cell is equal to the reduction

  potential plus the oxidation potential:

  Eocell = Eored + Eoox

  In this chemical reaction, Au3+ is reduced to Au making this the reduction half-reaction (Eored= + 1.55 V). Oppositely, Br?is oxidized to Br2, so this would be the oxidation half-reaction.

  Therefore, you must reverse the reduction potential given for Br2 so that it matches the chemical reaction given, as follows:

  2Br?aq) Br2 (l) + 2e?Eoox= ?Eored= -(+1.07 V) = -1.07 V

  Therefore, the Eocell = Eored + Eoox = (+1.55 V) + (-1.07 V) = + 0.48 V, making D the correct answer.

  Note:

  When calculating the cell potentials, do not multiply the reduction potential or oxidation potential by the coefficients.

• Question 73:

  How many electrons can fit into the third shell of an atom?

  A. 16

  B. 4

  C. 2

  D. 18

  Correct Answer: D

  The first shell can contain only 2 electrons, the second shell can hold up to 8, and third can contain up to 18.

• Question 74:

  How will the gravitational force between two objects change if the distance between them is doubled?

  A. It will remain constant.

  B. It will decrease by a factor of 4.

  C. It will increase by a factor of 4.

  D. It will be cut in half.

  Correct Answer: B

  The gravitational force between two objects is inversely proportional to the distance between them squared.

• Question 75:

  TMS (tetramethylsilane) is commonly added to nuclear magnetic resonance (NMR) samples. It has a chemical shift which is upfield of most organic signals, and it gives a strong, sharp singlet peak and can be easily removed. Why is TMS added to NMR samples?

  A. to reduce noise in the spectrum

  B. to calibrate the NMR spectrum

  C. to react with the sample

  D. to provide a lock signal

  Correct Answer: B

  The sharp and distinct peak of TMS is used as a standard from which NMR spectra can be calibrated.

• Question 76:

  Which observation is consistent with the Doppler effect?

  A. An ambulance siren sounds high pitched as the ambulance moves away from you.

  B. The absorption spectrum of light emitted from a star moving away from Earth appears to be blue.

  C. The absorption spectrum of light emitted from a star moving towards Earth appears to be red.

  D. An ambulance siren sounds high pitched as the ambulance moves towards you.

  Correct Answer: D

  According to the Doppler effect, as a source moves towards an observer, the wavelength decreases and the frequency increases. When a source moves away from an observer, the wavelength increases and the frequency decreases.

• Question 77:

  

  A star with a surface temperature of 20,000o and a low luminosity would most likely be classed as which kind of star?

  A. giant

  B. supergiant

  C. main sequence star

  D. white dwarf

  Correct Answer: D

  A Hertzprung-Russell diagram plots the temperatures of stars against their luminosities and can be used to categorize them. According to the diagram, a star with a low luminosity and a high temperature would most likely be classed as a White Dwarf.

• Question 78:

  

  A Hertzsprung - Russel diagram illustrates the different stages in a star's life cycle. Stars evolve along the main sequence, increasing in surface temperature. After the main sequence stage, a star cools and expands. Using the diagram, which stage would you expect to come directly after the main sequence?

  A. giant

  B. supernova

  C. white dwarf

  D. none of these

  Correct Answer: A

  The diagram shows that compared to main sequence stars, giant or supergiant stars tend to have lower surface temperatures and are larger. The most massive main sequence stars will evolve into supergiants, while lower mass main sequence stars, like the sun, will evolve into giants.

• Question 79:

  You are standing next to a river which is 10 m wide and you may assume that the profile of the river bed is a semicircle. What other piece of information do you need to estimate the volume of water that will flow past a 1-meter stretch of the river within 24 hours?

  A. the density of the water

  B. the speed of the water

  C. more than one piece of additional information is required

  D. the total length of the river

  Correct Answer: B

  You can estimate the volume of water in the 1-meter cross section by calculating the area of the semicircular river bed and multiplying by the length of the stretch of river. If you know the speed of water flow, you can determine how many seconds it will take for this volume of water to travel across the 1-meter stretch. Divide 24 hours by this time and multiply the result by the cross-sectional volume to get the total volume in 24 hours.

• Question 80:

  Which type of isomerism is shown by the following compounds?

  CH3-CH2-O-CH2-CH3 CH3-CH2-CH2-O-CH3

  A. Position isomerism

  B. Chain isomerism

  C. Metamerism

  D. Functional isomerism

  Correct Answer: C