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Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 11 )

Updated On: 12-Jan-2026
Page 11 of 164
PDF with 811 Questions

The lead-acid battery, also called a lead storage battery, is the battery of choice for starting automobiles. It contains 6 cells connected in series, each composed of a lead oxide cathode "sandwiched" between 2 lead anodes. Insulating separators are placed between the electrodes to prevent internal short-circuits. Aqueous sulfuric acid is the electrolyte.
When the battery is being discharged, the following reaction takes place:
Pb(s) + PbO2(s) + 2H2SO4 (aq) 2PbSO4 (s) + 2H2O
Reaction 1
The electrode reactions, both written as reductions, are shown in Table 1.
Table 1

As a car operates, the battery is recharged by electricity produced by the car's alternator, an AC generator whose ultimate power source is the car's internal combustion engine. In spite of this, batteries eventually lose their power. The battery is said to be "dead" when Reaction 1 has proceeded completely to the right.
Which of the following occurs as the battery is being recharged?

  1. An increase in the concentration of H+ ions
  2. An increase in the concentration of PbSO4 and Pb
  3. An increase in the concentration of H2O
  4. An increase in the concentration of PbSO2

Answer(s): A

Explanation:

The battery is being recharged, so Reaction 1 is proceeding to the left. Since aqueous sulfuric acid, H2SO4, is one of the products of recharging, the concentration of H+ will increase, making choice A the correct answer.
Choice B is wrong because PbSO4(s) is serving as a reactant during recharging, and is therefore consumed during the reaction. Choice C is wrong because water is also a reactant during recharging. Choice D is wrong
because the amount of lead oxide increases, not decreases as the battery is being recharged.



The lead-acid battery, also called a lead storage battery, is the battery of choice for starting automobiles. It contains 6 cells connected in series, each composed of a lead oxide cathode "sandwiched" between 2 lead anodes. Insulating separators are placed between the electrodes to prevent internal short-circuits. Aqueous sulfuric acid is the electrolyte.
When the battery is being discharged, the following reaction takes place:
Pb(s) + PbO2(s) + 2H2SO4 (aq) 2PbSO4 (s) + 2H2O
Reaction 1
The electrode reactions, both written as reductions, are shown in Table 1.
Table 1

As a car operates, the battery is recharged by electricity produced by the car's alternator, an AC generator whose ultimate power source is the car's internal combustion engine. In spite of this, batteries eventually lose their power. The battery is said to be "dead" when Reaction 1 has proceeded completely to the right.
The graph below shows the change in potential versus time of a 12-V lead storage battery during discharge.

Which of the following is true?

  1. The electrolyte density at point A is greater than it is at point B.
  2. The electrolyte density at point A is less than it is at point
  3. The electrolyte density at point A is the same as it is at point B.
  4. The electrolyte density at points A and B cannot be compared without more information.

Answer(s): B

Explanation:

As can be seen in Reaction 1, when the lead-acid battery is being discharged, sulfuric acid, a reactant, is being consumed. Since it is stated in the passage that sulfuric acid is the electrolyte, the density will decrease as the discharge progresses, so the density at point A is less than that at point B. Choice B is, therefore, the correct answer. Choice A is wrong because at point A there is less electrolyte present, not more. Choice C is wrong because the density decreases as the discharge progresses. Choice D is wrong because there is a direct relationship between the density of the electrolyte and the state of discharge.



The lead-acid battery, also called a lead storage battery, is the battery of choice for starting automobiles. It contains 6 cells connected in series, each composed of a lead oxide cathode "sandwiched" between 2 lead anodes. Insulating separators are placed between the electrodes to prevent internal short-circuits. Aqueous sulfuric acid is the electrolyte.
When the battery is being discharged, the following reaction takes place:
Pb(s) + PbO2(s) + 2H2SO4 (aq) 2PbSO4 (s) + 2H2O
Reaction 1
The electrode reactions, both written as reductions, are shown in Table 1.
Table 1

As a car operates, the battery is recharged by electricity produced by the car's alternator, an AC generator whose ultimate power source is the car's internal combustion engine. In spite of this, batteries eventually lose their power. The battery is said to be "dead" when Reaction 1 has proceeded completely to the right.
Often in cold weather the battery goes "dead". Thermodynamic data confirms that the voltage of most electrochemical cells decreases with decreasing temperature. If the battery is warmed to room temperature, it often recovers its ability to deliver normal power. The battery appeared "dead" because:
I) the resistance of the electrolyte had decreased.
II) the viscosity of the electrolyte had increased.
III) the viscosity of the electrolyte had decreased.

  1. I only
  2. II only
  3. I and II only
  4. I and III only

Answer(s): B

Explanation:

The reason that the battery "goes dead" with decreasing temperature is that the viscosity of the electrolyte increases. At higher viscosities the ions are moving much slower, which leads to an increase in resistance and a decrease in the power output. Roman numeral I states that the resistance of the electrolyte has decreased.
This is not true, the resistance of the electrolyte increases with decreasing temperature. Since choice A, choice C, and choice D all contain Roman numeral I, they can be eliminated, leaving choice B as the correct answer.



A helium-neon gas discharge laser as shown in Figure 1 below generates a coherent beam of monochromatic light at a wavelength of 632.8 nm.

Figure 1
A discharge current of electrons is created in the tube by an applied voltage. When these electrons collide with the helium atoms, they can excite ground-state helium electrons to an energy level of 20.61 eV. The excited electrons cannot decay back to the ground state by emitting a photon because such a transition does not conserve angular momentum. Instead, if the excited helium atom collides with a neon atom, a ground-state electron in the neon atom can be excited to an energy level of 20.66 eV, and the helium electron can return to its ground state.
The above process occurs quite often in the tube until the percentage of neon atoms with electrons in the 20.66-eV energy level is greater than the percentage of neon atoms with electrons in lower levels. This condition is called a population inversion. An excited electron in one of the neon atoms can then spontaneously decay by emitting a photon of wavelength 632.8 nm in a random direction. The photon will stimulate the same transition in another excited electron in a neon atom. The photon radiated by this stimulated emission process travels in the same direction as the original photon. The resulting light is then reflected back and forth inside the tube until it escapes through the partially transparent mirror. (Note: A photon's energy in eV is given by E = 1240/, where is the photon's wavelength in nm. The helium and neon ground-state energies are both 0 eV.)
A laser produces light with a wavelength of 200 nm at a power of 6.2 × 1015 eV/s. How many photons per second does this laser deliver?

  1. 1.0 × 1015
  2. 2.0 × 1015
  3. 4.0 × 1015
  4. 10.0 × 1015

Answer(s): A

Explanation:

The power of the laser is 6.2 × 1015 eV/s, which means that the laser produces 6.2 × 1015 eV of energy each second. This energy takes the form of photon energy. To solve for the number of photons produced per second, figure out how many photons correspond to an energy of 6.2 × 1015 eV. The formula given in the passage states that the energy (in eV) of one photon is given by E = 1240/, where is the photon's wavelength in nm. Hence, the energy of n photons must given by n(1240/). Setting this expression equal to 6.2 × 1015 eV and solving for n, we obtain n = (6.2 × 1015 / 1240). The question stem states that for this laser, = 200 nm.
Substituting in, we calculate n = 200(6.2 × 1015 / 1240) = 1.0 × 1015, which is choice A.



A helium-neon gas discharge laser as shown in Figure 1 below generates a coherent beam of monochromatic light at a wavelength of 632.8 nm.

Figure 1

A discharge current of electrons is created in the tube by an applied voltage. When these electrons collide with the helium atoms, they can excite ground-state helium electrons to an energy level of 20.61 eV. The excited electrons cannot decay back to the ground state by emitting a photon because such a transition does not conserve angular momentum. Instead, if the excited helium atom collides with a neon atom, a ground-state electron in the neon atom can be excited to an energy level of 20.66 eV, and the helium electron can return to its ground state.
The above process occurs quite often in the tube until the percentage of neon atoms with electrons in the 20.66-eV energy level is greater than the percentage of neon atoms with electrons in lower levels. This condition is called a population inversion. An excited electron in one of the neon atoms can then spontaneously decay by emitting a photon of wavelength 632.8 nm in a random direction. The photon will stimulate the same transition in another excited electron in a neon atom. The photon radiated by this stimulated emission process travels in the same direction as the original photon. The resulting light is then reflected back and forth inside the tube until it escapes through the partially transparent mirror. (Note: A photon's energy in eV is given by E = 1240/, where is the photon's wavelength in nm. The helium and neon ground-state energies are both 0 eV.)
Why is stimulated emission of photons necessary in order to produce a coherent beam of light instead of spontaneous emission alone?

  1. Stimulated emission produces photons of higher energy than those produced by spontaneous emission.
  2. Stimulated emission produces photons that travel in the same direction as the photon that induces their emission.
  3. Stimulated emission produces photons with longer wavelengths than those produced by spontaneous emission.
  4. Either spontaneous or stimulated emission alone would be sufficient to produce laser light.

Answer(s): B

Explanation:

To answer this question, you have to figure out the difference between stimulated and spontaneous emission.
The passage states that a photon is emitted in a random direction when an atom spontaneously decays. This process is called spontaneous emission. It also states that a photon can stimulate an electron transition in an atom. The photon that is emitted in this process, called stimulated emission, travels in the same direction as the stimulating photon. Therefore, spontaneous emission produces photons that travel in random directions, whereas stimulated emission produces photons that travel in the same direction as the stimulating photon. A coherent beam of light consists of photons travelling in the same direction. So choice B is correct.
Choices A and C are wrong because, as stated in the third paragraph, the photon produced by spontaneous emission causes stimulated emission by inducing the same electron transition in another excited atom. Since the electron transition is the same, the photon energy released by the transition is the same, and the photon wavelengths must be the same because energy and wavelength are related by the formula E = 1240/.
Choice D is incorrect because stimulated emission is necessary to obtain a large number of photons traveling in the same direction.



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