Medical Tests Medical Tests Certifications MCAT-TEST Questions & Answers

• Question 221:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

  

  CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1 How are the values of Pe calculated?

  

  A. (T/273)(1 atm)

  B. (T/298)(1 atm)

  C. [(T ?273)/273](1 atm)

  D. [(T ?298)/298](1 atm)

  Correct Answer: B

  Pe as the table explains, is the expected pressure at each temperature if no reaction had taken place. When the carbon dioxide is introduced into the furnace chamber, it is at a pressure of 1 atmosphere and a temperature of 298 K. Above 298 K, Pe is greater than 1 atmosphere because, at constant volume, the pressure increases as the temperature increases. Since Pe is the pressure if no reaction had taken place, any change in the number of moles of gas present does not have to be considered. Since the pressure of a gas is directly proportional to its temperature in Kelvins, the ratio of the EXPECTED pressure to the original pressure of 1 atmosphere must be equal to the ratio of the temperature of the trial to the original temperature of 298 K. The equation is as follows:

  Pe /(1 atm) = T/298

  Solving for Pe yields answer choice B.

• Question 222:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

  

  CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1

  

  If 0.6 g of elemental carbon are consumed during a trial, how many grams of CO are produced?

  A. 0.8

  B. 1.4

  C. 1.6

  D. 2.8

  Correct Answer: D

  Since carbon has a molar mass of 12 grams, a tenth of a mole would be 1.2 grams, and the 0.6 grams stated in the question is one twentieth of a mole. The equation given in the passage shows that for every one mole of carbon consumed, two moles of CO are produced. So, if one twentieth of a mole of carbon is consumed, two twentieths, or one tenth, of a mole of CO will be produced. One tenth of the molar mass of CO, 28 grams, is 2.8 grams. Therefore, choice D is the correct response.

• Question 223:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below: CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1

  

  

  Which of the following is NOT necessarily true about the equilibrium reaction between CO2, C, and CO?

  A. The standard entropy change is positive.

  B. A decrease in pressure at constant temperature would shift the equilibrium to the right.

  C. Addition of CO will shift the equilibrium to the left.

  D. The standard Gibbs' free energy change is negative.

  Correct Answer: D

  You should know the relationship G = H ?TS. S for this reaction is positive, because there are more moles of gaseous products than reactants, but there is no information about the sign of H. Since H cannot be determined, neither can G. Choice D is the correct response. Choice A is wrong (that is, the statement is certainly true) because, as was just stated, the entropy change is positive. Choice B is wrong because, according to Le Chatelier's principle, a decrease in pressure will favor the side of the reaction with more moles of gas. In this case, it is the products side, and the equilibrium is said to have shifted to the right. Choice C is wrong because, again, according to Le Chatelier's principle, addition of a product will shift the equilibrium to the left, that is, the reactants side.

• Question 224:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

  

  CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1

  

  Which of the following shows the correct Lewis structure of carbon monoxide?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  Correct Answer: C

  The first thing that should be done is to determine which choices have the proper number of valence electrons. Carbon monoxide must have a total of 10 electrons: six from the oxygen and four from the carbon. Choice B and choice D can be

  eliminated because they both have 12 electrons.

  In choice A, the carbon has one pair of unbonded electrons, plus four more electrons in the double bond. This gives the carbon a total of only six electrons in its valence shell, so the carbon doesn't have a complete octet, and choice A must

  be wrong. Choice C shows the correct Lewis structure for carbon monoxide: each atom has a complete octet, and there is a total of ten electrons.

• Question 225:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

  

  CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1 When the system stabilized at 1,200 K, a sample of helium was injected into the furnace. What should happen to the amount of carbon dioxide in the system?

  

  A. It should increase.

  B. It should decrease.

  C. It should be completely converted to carbon monoxide.

  D. It should remain the same.

  Correct Answer: D

  You should know that helium, a noble gas, is very unreactive and would almost certainly not react with any of the species in the furnace. Because the helium does not react with any of the species that participate in the equilibrium, the equilibrium is unaffected by the addition of helium. Even though it increases the total pressure inside the system, the partial pressures of the reacting gases are unchanged (Dalton's law) and therefore they keep on behaving as if the helium weren't present. The correct choice is therefore choice D.

• Question 226:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

  

  CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1

  

  What can be said about the value of S?of the reaction?

  A. It is positive.

  B. It is negative.

  C. It is zero.

  D. It cannot be determined from the information given.

  Correct Answer: A

  The entropy of a reaction is a measure of the disorder of the system. The sign of the value of the entropy indicates whether the products of a reaction are more or less disordered than the reactants. A reaction that experiences an increase in disorder in going from reactants to products would have a positive S for the reaction. Similarly, a reaction that decreases its disorder would have a negative S for the reaction. In this reaction, 1 mole of carbon dioxide, a gas, reacts with 1 mole of solid carbon to give two moles of carbon monoxide gas. Since there are more moles of gas in the products than in the reactants, there is an increase in entropy, meaning that the sign of the change is positive. Choice A is therefore the correct response.

• Question 227:

  A researcher investigated the equilibrium between CO2, C, and CO as a function of temperature. The equation is given below:

  

  CO2(g) + C(s) 2 CO(g) Carbon dioxide, at 298 K and 1 atm, and an excess of powdered carbon were introduced into a furnace, which was then sealed so that pressure would increase as the temperature rose. The furnace was heated to, and held constant at, a predetermined temperature. The pressure within the furnace chamber was recorded after it had remained unchanged for one hour. The table below shows the pressures recorded for a series of temperatures together with the pressures expected if no reaction had taken place. Table 1

  

  How many pi bonds are in the carbon dioxide molecule?

  A. 0

  B. 1

  C. 2

  D. 3

  Correct Answer: C

  The carbon dioxide molecule has a central carbon bonded to two oxygen atoms: carbon shares two electrons with each oxygen, and each oxygen shares two electrons with carbon. As a result of this sharing pattern, two double bonds are formed. You should know that a double bond consists of one sigma bond and one pi bond. Since there are two double bonds, there are two pi bonds, making choice C the correct choice.

• Question 228:

  There are two opposing theories of light: the particle theory and the wave theory. According to the particle theory, light is composed of a stream of tiny particles that are subject to the same physical laws as other types of elementary particles.

  One consequence of this is that light particles should travel in a straight line unless an external force acts on them. According to the wave theory, light is a wave that shares the characteristics of other waves. Among other things, this means

  that light waves should interfere with each other under certain conditions.

  In support of the wave theory of light, Thomas Young's double slit experiment proves that light does indeed exhibit interference. Figure 1 shows the essential features of the experiment. Parallel rays of monochromatic light pass through two

  narrow slits and are projected onto a screen. Constructive interference occurs at certain points on the screen, producing bright areas of maximum light intensity. Between these maxima, destructive interference produces light intensity minima.

  The positions of the maxima are given by the equation dsin = n, where d is the distance between the slits, is the angle shown in Figure 1, the integer n specifies the particular maxima, and is the wavelength of the incident light. (Note: sin tan

  for small angles.).

  

  Figure 1

  According to the modern theory of light, a beam of light may be described either as a stream of particles or as a wave, depending on the circumstances. Which of the following correctly states a connection between the two descriptions?

  A. The number of light particles that pass by per second is proportional to the frequency of the light wave.

  B. The mass of each particle of light is proportional to the intensity of the light wave.

  C. The size of each particle of light is proportional to the wavelength of the light wave.

  D. The energy of each particle of light is proportional to the frequency of the light wave.

  Correct Answer: D

  A particle of light is known as a photon. You should know the formula E = hf, which states that the energy E of a photon is equal to Planck's constant h times the frequency f of the light. The photon energy is thus proportional to the frequency of the light. Since the photon energy is a property of a particle and frequency is a property of a wave, this relationship successfully connects the particle and wave descriptions of light, and choice D is correct.

  Choice A is incorrect because the number of particles passing by per unit time is related to the intensity of the light, not its frequency. Choice B is wrong because photons have zero mass. Choice C is wrong because photons are thought of as point-like particles without size.

• Question 229:

  There are two opposing theories of light: the particle theory and the wave theory. According to the particle theory, light is composed of a stream of tiny particles that are subject to the same physical laws as other types of elementary particles.

  One consequence of this is that light particles should travel in a straight line unless an external force acts on them. According to the wave theory, light is a wave that shares the characteristics of other waves. Among other things, this means

  that light waves should interfere with each other under certain conditions.

  In support of the wave theory of light, Thomas Young's double slit experiment proves that light does indeed exhibit interference. Figure 1 shows the essential features of the experiment. Parallel rays of monochromatic light pass through two

  narrow slits and are projected onto a screen. Constructive interference occurs at certain points on the screen, producing bright areas of maximum light intensity. Between these maxima, destructive interference produces light intensity minima.

  The positions of the maxima are given by the equation dsin = n, where d is the distance between the slits, is the angle shown in Figure 1, the integer n specifies the particular maxima, and is the wavelength of the incident light. (Note: sin tan

  for small angles.)

  

  Figure 1

  Light waves can be described in terms of frequency f and wavelength or in terms of wave number k and angular frequency . These quantities are related by the following equations:

  k = 2/ and = 2f

  Which equation below accurately describes the speed of the wave v in terms of k and ?

  A. v = f

  B. v = + k

  C. v = /k

  D. v = k

  Correct Answer: C

  You should know that the speed v of a wave is given by v = f, where f is the frequency of the wave, and is its wavelength. This makes sense, since is the distance traveled by the wave in one cycle, and frequency is the inverse of the period, which is the time it takes for one wave to cycle. Now solve the relation in the question stem k = 2/ for to obtain = 2/k. Also solve = 2f for f to obtain f = /2. Substituting these expressions into the speed equation, we obtain v = f = (/2)(2/k) = /k, which is choice C. Choice A, while a correct relation, is not the right answer because it does not express v in terms of and k. Choices B and D can be eliminated on the basis of dimensional analysis because they do not have the units of speed, m/s. k has units of m-1 and has units of S-1, so choice B is nonsensical and choice D has units of 1/(ms).

• Question 230:

  There are two opposing theories of light: the particle theory and the wave theory. According to the particle theory, light is composed of a stream of tiny particles that are subject to the same physical laws as other types of elementary particles.

  One consequence of this is that light particles should travel in a straight line unless an external force acts on them. According to the wave theory, light is a wave that shares the characteristics of other waves. Among other things, this means

  that light waves should interfere with each other under certain conditions.

  In support of the wave theory of light, Thomas Young's double slit experiment proves that light does indeed exhibit interference. Figure 1 shows the essential features of the experiment. Parallel rays of monochromatic light pass through two

  narrow slits and are projected onto a screen. Constructive interference occurs at certain points on the screen, producing bright areas of maximum light intensity. Between these maxima, destructive interference produces light intensity minima.

  The positions of the maxima are given by the equation dsin = n, where d is the distance between the slits, is the angle shown in Figure 1, the integer n specifies the particular maxima, and is the wavelength of the incident light. (Note: sin tan

  for small angles.)

  

  Figure 1

  Which of the following is sufficient information to determine the approximate speed of a ray of light in water?

  A. The angle of incidence and the angle of refraction of the light ray as it enters water from air

  B. The wavelength in water and the wavelength in air of the light ray as it enters water from air

  C. The speed of light in a vacuum and the density of water

  D. The speed of light in a vacuum and the index of refraction of water

  Correct Answer: D

  You should know that the speed of light is different in different media. The formula associated with this concept is n = c/v, where n is the index of refraction of a given medium, c is the speed of light in a vacuum, and v is the speed of light in the given medium. Hence, if the index of refraction and the speed of light in a vacuum are known, we can solve for the speed of light in the medium. Thus, choice D is correct.

  We would need the air's index of refraction, along with the variables in choice A, to calculate the water's index of refraction using Snell's law. Even if we could calculate the water's index of refraction, we would need the speed of light in a vacuum to calculate the speed of light in water, so choice A is wrong. Similarly, the variables in choice B allow the calculation of the index of refraction, but the speed of light is still required so choice B is wrong. Choice C is incorrect because while density might play an indirect role in the index of refraction, the density is not sufficient information to determine the index of refraction.