Medical College Admission Test (MCAT) Physics Sample Questions |
Passage I
A series of chemical reactions was carried out to study the chemistry of lead.
Reaction 1
Initially, 15.0 mL of 0.300 M Pb(NO3)2(aq) was mixed with 15.0 mL of 0.300 M Na2SO4(aq).All the Pb(NO3)2 reacted to form Compound A, a white precipitate.Compound A was removed by filtration.
Reaction 2
Next, 15.0 mL of 0.300 M KI(aq) was added to Compound A. The mixture was agitated and some of Compound A dissolved. In addition, a yellow precipitate of PbI2(s) was formed.
Reaction 3
The PbI2(s) was separated and mixed with 15.0 mL of 0.300 M Na2CO3(aq). A white precipitate of PbCO3(s) formed. All of the PbI2(s) was converted into PbCO3(s).
Reaction 4
The PbCO3(s) was removed by filtration and a small sample gave off a gas when treated with dilute HCl.
Some sample questions on this passage are as follows:
Answer: A
Explanation: Reaction 4 is shown in the following equation, which is answer choice A.
PbCO3(s) + 2 HCl(aq) = PbCl2(aq) + CO2(g) + H2O(l)
Answers B and D do not show a reaction involving PbCO3(s), as required by Reaction 4. Answer C shows an implausible and unbalanced equation. Thus, answer choice A is the best answer.
Answer: D
Explanation: Reaction 1 is shown in the following equation.
Pb(NO3)2(aq) + Na2SO4(aq) = PbSO4(s) + 2 NaNO3(aq)
Compound A, the white solid, is PbSO4(s). Neither the reactant Pb(NO3)2 nor the product NaNO3 can precipitate because all nitrates and sodium salts are water soluble. PbI2 cannot precipitate because iodide is not present. Thus, answer choice D is the best answer.
Answer: A
Explanation: The dissolution of Pb(OH)2(s) is represented by the following equation.
Pb(OH)2(s)=Pb2(aq) + 2 OH-(aq)
At pH 9, the concentration of OH-(aq) is greater than the concentration of OH-(aq) at pH 7. According to Le Châtelier's principle, the additional common ion, OH-(aq), will shift the position of equilibrium to the left, and less Pb(OH)2 will dissolve. Thus, answer choice A is the best answer.
Answer: B
Explanation: The reactions described in the passage show that lead(II) is successively precipitated as PbSO4, PbI2, and PbCO3. This sequence shows (assuming equal anion concentrations, as must be done here) that PbCO3 is less soluble than PbI2, and PbI2 is less soluble than PbSO4. The order in which the anions precipitate Pb2- is: CO32- then I- then SO42-. When this sequence is applied to the question, answer choice B is in the correct order, and answers A, C, and D are all in the opposite order. Thus, answer choice B is the best answer.
Answer: B
Explanation: The initial Na2SO4(aq) solution in Reaction 1 is 15 mL of 0.300 MNa2SO4(aq).
(15.0 mL) ( 1 L ) (0.300 mol Na2SO4) (2 mol Na+)
(1000 mL) (1 L Na2SO4(aq) ) (1 mol Na2SO4)
= 0.00900 mol = Answer B
Passage II
At the critical point, the density of liquid CO2 is equal to the density of gaseous CO2.This occurs at specific conditions of temperature and pressure.At temperatures and pressures above the critical point values, CO2 is deemed supercritical.For a supercritical fluid, the density and ability to dissolve other substances are similar to values expected for liquids.The following figures give phase data for CO2.
Figure 1 Phase diagram for CO2
Some sample questions on this passage are as follows:
Answer: D
Explanation: The liquid and vapor phases coalesce at point D of Figure 2, where the densities of liquid and gaseous CO2are equal. Thus, answer choice D is the best answer
Answer: D
Explanation:The question does not compare CO2 to a specific solvent, so we are looking for an inherent property of CO2 that makes it a good solvent for an organic oil. Supercritical CO2 is similar to a liquid and can be easily removed by evaporation because it changes into a gas when the pressure is lowered. Answers A, B, and C are not true of CO2, and answers A and C are not desirable properties of an extraction solvent. Thus, answer choice D is the best answer
Answer: D
Explanation: Polar water molecules are held together by relatively strong hydrogen bonds; whereas, the linear, nonpolar molecules of CO2 are held together at room temperature by weak London dispersion forces. Thus, answer choice D is the best answer.
Answer: C
Explanation:The critical point, shown as a dot (·) in Figure 1, is near 30oC and 80 atm. Answer choice C is the best answer
Answer: B
Explanation: According to the principle of “like dissolves like,” the covalent compound CO2 is a better solvent for a covalent compound than it is for an ionic compound. Diethyl ether, C2H5OC2H5, is a covalent compound and NaCl, NH4NO3, and KOH are ionic compounds. Thus, answer choice B is the best answer.
Passage III
When X-rays are produced in an X-ray tube, two types of X-ray spectra are observed: continuous spectra and line spectra.
A continuous spectrum is produced by bremsstrahlung, the electromagnetic radiation produced when free electrons are accelerated during collisions with ions.
A line spectrum results when an electron having sufficient energy collides with a heavy atom, and an electron in an inner energy level is ejected from the atom.An electron from an outer energy level then fills the vacant inner energy level, resulting in emission of an X-ray photon.For example, if an electron in the n = 1 energy level is ejected from an atom, an electron in the n = 2 level of the atom can fill the vacancy created in the n = 1 level, and a photon with an energy equal to the energy difference between the two levels will be emitted.
A scientist produced both types of spectra using the X-ray tube shown in Figure 1 below.
Figure 1 Heated cathode X-ray tube
The tube contains a heated filament cathode (C), which emits electrons.A power supply (LV) regulates the filament temperature, the electrical current in the tube, and the number of X-rays produced at the anode (A).Another power supply (HV) regulates electron acceleration.
The scientist used an X-ray tube to determine the relationship between X-ray wavelength, ?, and X-ray intensity, I, which is proportional to the number of X-ray photons emitted at ?.The scientist then graphed the results of the experiment, as shown in Figure 2.
Figure 2 X-ray intensity versus wavelength
Some sample questions on this passage are as follows:
Answer: C
Explanation: The emission peaks P1 and P2 are described in the passage as due to an electron from an outer energy level filling a vacant inner energy level, resulting in emission of an X-ray photon. These photons have discrete energies, and therefore discrete wavelengths, so they appear in the spectrum as peaks. Thus, answer choice C is the best answer.
Answer: A
Explanation: The power P, supplied by the battery to accelerate the electron beam is given by the formula P = I·V, where I is the beam current and V is the potential difference between the cathode and anode. Therefore P = (5 x 10-3 A) x (105 V) = 5 x 102 W. Therefore, answer choice A is the best answer.
Answer: C
Explanation: The emitted X-ray has the positive energy difference between the atomic energy levels as measured by their ionization potentials. For Pb n = 2 to 1, that is (1,400 x 10-17 J) - (240 x 10-17 J) = 1160 x 10-17 J = 1.16 x 10-14 J. Thus, answer choice C is the best answer.
Answer: A
Explanation: According to the passage, bremsstrahlung is produced when electrons are accelerated during collisions with ions. All the choices of answers are ions except He, a neutral atom. Therefore, answer choice A is the correct answer.
Answer: A
Explanation: To increase the kinetic energy of the electrons, they must be accelerated by a higher voltage between the cathode and anode, thus the voltage of HV was increased. Thus, answer choice A is the best answer.
Answer: D
Explanation: The probability of an X-ray emission event at a given wavelength is measured by its intensity in the spectrum. In Figure 2, P2 has a higher intensity than does P1. Thus, answer choice D is the best answer
Passage IV
Many chemical bonds are not purely ionic or covalent, but polar covalent.For example, in an HCl bond, chlorine has a greater attraction for electrons than hydrogen does and therefore develops a partial negative charge with respect to the hydrogen atom.
If the partial charges are separated by a known distance, the dipole moment, a measure of the charge separation in a bond or molecule, can be calculated by the following equation.
dipole moment = charge x separation distance
Dipole moments are usually measured in debyes (D), where 1 D = 3.34 x 10-30 coulomb · meter.
The molecular geometry of some simple molecules can be determined based on the presence or absence of a net dipole moment in the molecule.The observed molecular dipole moments of various compounds are reported in Table 1.
Table 1 Dipole Moments for Various Compounds
| Molecule | Dipole moment (D) |
| SnBr4 | 0 |
| HgBr2 | 0 |
| SO2 | 1.47 |
| HF | 1.82 |
| HNO3 | 2.17 |
| BaO | 7.95 |
| KCl | 10.27 |
Some sample questions on this passage are as follows:
Answer: D
Explanation: The dipole moment of a molecule is the vector sum of all of the bond moments. According to the data in Table 1, the dipole moment of SnBr4 is zero; therefore, its bond moments add to zero or cancel. Thus, answer choice D is the best answer.
Answer: B
Explanation: Table 1 gives the dipole moment of HF as 1.82 D. Chlorine is just below fluorine in the periodic table; therefore, the electronegativity of chlorine, though significant, is less than that of fluorine. Chlorine is less effective than fluorine in creating a separation of charge when bonded to hydrogen, and the dipole moment of HCl is slightly less than that of HF. Thus, answer choice B is the best answer.
Answer: D
Explanation: HCl is polar covalent because H and Cl share a pair of bonded electrons that are more strongly attracted to the chlorine atom. The higher effective nuclear charge (i.e., the charge of the nucleus minus the shielding caused by extranuclear electrons) of chlorine accounts for its greater electronegativity. Thus, answer choice D is the best answer.
Answer: D
Explanation: Carbon dioxide,O=C=O, is linear. Therefore, the two CO dipoles cancel because they are in opposite directions. If one of the oxygen atoms is removed, the resulting CO will have a dipole because the species is linear and comprised of two different atoms. Thus, the dipole moment will change from zero in CO2 to a positive value in CO. Thus, answer choice D is the best answer
Answer: B
Explanation: An analysis of the two structures shows that the bond moments in PCl5 add to zero; whereas, those in PCl3 do not. As shown in the figure, PCl3 is pyramidal not planar. Thus, answer choice B is the best answer.
Answer: C
Explanation: Work is the product of the force on an object and the distance the object moves in the direction of the applied force. In this case, work = 20 N x 10 m = 200 J. Thus, answer choice C is the best answer.
Answer: B
Explanation: Evaporation occurs when a molecule attains sufficient speed or kinetic energy to overcome the attractive forces of a liquid. Resonance, surface tension and potential energy all relate to molecules that are not in motion. Thus, answer choice B is the best answer.
Answer: C
Explanation: The relation between distance, acceleration, and time is: d = (1/2)a · t2. To solve for the time it takes the runner to use t = (2d/a)1/2 = (2×3/1.5)1/2 = 2 s. Therefore, answer choice C is the best answer.
A third force, F, is applied to the center of the sheet, along a line in the plane of the sheet, at an angle ? = arctan 0.75 with respect to the horizontal direction.The sheet will be in translational equilibrium when F has what value?
Answer: C
Explanation: A body is in transitional equilibrium when the components of all external forces cancel. For the sheet: F cos ? = 4 N, F sin? = 3 N. The magnitude of F is found by adding the squares of the components: F2cos2 ? + F2 sin2 ?= F2 = 42 + 32 = 25 N2. Therefore F = 5 N. The F vector points in the proper direction since tan ? = 0.75 = 3/4. Thus, answer choice C is the best answer.
Answer: C
Explanation: In general, catalysts lower the activation energy of the slowest step in a reaction. Thus, they increase the rate of the reaction without increasing the number of collisions, the kinetic energy of the reactants, or the Keq of a reversible reaction. Thus, answer choice C is the best answer.
Answer: B
Explanation: Conservation of linear momentum requires: mradonvradon= mheliumvhelium with helium identified as the alpha particle. The nuclear masses can be approximated by their mass numbers (222 and 4). Thus, the recoil speed of the radon is (4/222)×1.5 x 107 m/s = 2.7 x 105 m/s. Therefore, answer choice B is the best answer.
Answer: A
Explanation: The ratio of object to image distance equals the ratio of object to image height. The ratio of image to object height is found by rearranging the ratios to give 4f /(4/3)f = 1/3. The image is demagnified by a factor of 3. Thus, answer choice A is the best answer.
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