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MCAT Sample Questions : Physical Sciences

  1. The standard potential for the reaction

    Zn2+ + 2 e- = Zn

    equals 0.763 V. When a strip of Zn is placed in a 0.1 M solution of HCl(aq), does the zinc strip begin to dissolve?

    1. Yes; H2(g) and ZnCl(aq) are produced.
    2. Yes; H2(g) and ZnCL2(aq) are produced.
    3. No; no reaction occurs because Zn is less reactive than is H2.
    4. No; no reaction occurs because Zn2+ is less reactive than is H+.

    Answer: B

    Explanation: As written, the equation shows the reduction of Zn2+ to Zn. The negative reduction potential for Zn2+ means that Zn has a positive oxidation potential (i.e., Zn is easily oxidized.) Zinc will displace hydrogen, which has a zero standard reduction or oxidation potential (i.e., hydrogen is the standard against which other substances are measured.). As given in the problem, ionic zinc is Zn2+. Therefore, zinc metal liberates hydrogen gas and produces ZnCL2(aq). Answer choice B is the best answer

  2. Which of the following must be known in order to determine the power output of an automobile?

    1. Final velocity and height
    2. Mass and amount of work performed
    3. Force exerted and distance of motion
    4. Work performed and elapsed time of work

    Answer: D

    Explanation: Power is defined as the rate of doing work. For the automobile, the power output is the amount of work done (overcoming friction) divided by the length of time in which the work was done. Therefore, answer choice D is the best answer

  3. In the molecule shown below, which bond is the longest?

    1. A
    2. B
    3. C
    4. D

    Answer: B

    Explanation: The bonds labeled C and D in the figure are of equal length but shorter than bond B. This is because two resonance structures can be drawn: in one resonance structure, bond C is a double bond, and in the second resonance structure, bond D is double bond. Double bonds (bond order = 2) are shorter than single bonds (bond order = 1), so the bond order for bonds C and D is about 1.5 and for bond B about 1. Bond B is longer than bond A because of the small atomic radius of hydrogen compared to nitrogen. Thus, answer choice B is the best answer.

    Alternatively, the figure shows that C = D, so answer choices C and D can be eliminated. Also, A is clearly shorter than B; therefore B is the longest. Thus, answer choice B is the best answer.

  4. A 15.0-eV photon collides with and ionizes a hydrogen atom. If the atom was originally in the ground state (ionization potential =13.6 eV), what is the kinetic energy of the ejected electron?

    1. 1.4 eV
    2. 13.6 eV
    3. 15.0 eV
    4. 28.6 eV

    Answer:

    Explanation: Conservation of energy requires that the 15.0 eV photon energy first provides the ionization energy to unbind the electron, and then allows any excess energy to become the electron's kinetic energy. The kinetic energy is this case is 15.0 eV-13.6 eV = 1.4 eV. Thus the correct answer is A.

  5. When 47Be undergoes radioactive decay by electron capture (a form of + decay), the resulting nucleus is:

    1. 36Li
    2. 37Li
    3. 47Be
    4. 48Be

    Answer: B

    Explanation: In radioactive decay, the sum of the mass numbers A and atomic numbers Z, before and after decay, must balance. The numbers for beryllium undergoing positron decay are: mass (7 = 7 + 0) and atomic (4 = 3 + 1). The resulting nucleus is 73Li. Thus, answer choice B is the best answer.

  6. A force F is used to raise a 4-kg mass M from the ground to a height of 5 m

    What is the work done by the force F? (Note: sin 60o = 0.87; cos 60o = 0.50. Ignore friction and the weights of the pulleys.)

    1. 50 J
    2. 100 J
    3. 174 J
    4. 200 J

    Answer: D

    Explanation: Work is the product of force and distance. The easiest way to calculate the work in this pulley problem is to multiply the net force on the weight mg by the distance it is raised: 4 kg x 10 m/s2 x 5 m = 200 J. Therefore, answer choice D is the best answer.

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