MCAT Physical : MCAT Physical Sciences
Study concepts, example questions & explanations for MCAT Physical
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Example Questions
Example Question #851 : Mcat Physical Sciences
Which of these statement is true regarding an emission spectrum?
The Hubble space telescope would observe absorption spectra from stars
The emission and absorption spectrum of an element show different wavelengths
As elements get heavier, their increasingly complex emission spectrum is simply added to the emission spectrum of the lighter elements
The emission spectrum of each element is unique
It is not possible to classify elements by their emission spectra because the spectra overlap so extensively
The emission spectrum of each element is unique
When energy is added to an atom, usually in the form of heat, the electrons may be displaced to higher quantum states; it is said that they "jump" to new orbital states. When they fall back to unexcited, or ground states, they emit exactly the amount of energy they took absorbed, either all at once, or in steps.
This emission is discrete, meaning that it creates light at very specific wavelengths. The emission spectrum is completely unique for each elemental atom, and the wavelengths are identical in the emission and absorption spectra, which can be thought of as photographic positive and negative images of exactly the same phenomenon.
Example Question #852 : Mcat Physical Sciences
Suppose an electron is excited from its ground state to 
A photon is emitted
The electron decreases in velocity
A photon is absorbed
The electron increases in velocity
A photon is absorbed
This question is testing your knowledge of what happens when an electron is excited. Electron excitation always accompanies an increase in energy, the opposite of which is equally true for electrons that fall back toward the ground state. Electron energy transitions are accompanied by electromagnetic energy consumption or release. When an electron increases in energy, it absorbs a certain unit of electromagnetic energy: a photon. When the electron decreases in energy, it emits the lost energy by releasing a photon.
Example Question #3 : Other Principles Governing Electrons
Photons can be generated with different wavelengths, depending on the number of energy levels available. What is the minimum number of different energy levels needed to produce six different wavelengths of light?
Three
Four
Six
Seven
Two
Four
From Bohr's model of the hydrogen atom, we know that as electrons jump to a higher energy state they become more excited, and absorb more energy. Additionally, we know that when an electron moves from a higher energy state to a lower energy state, it releases energy in the form of a photon. Because the emitted photon can be characterized by its wavelength, each jump between two different energy states will have a unique and different wavelength.
To produce six different wavelengths, we would need four different energy levels: n=1, n=2, n=3, n=4. The different wavelengths produced could be described by the following energy transitions.
1) 4 
2) 4 
3) 4 
4) 3 
5) 3 
6) 2 
Example Question #62 : Nuclear Chemistry And Electrons
Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
A scientist undertakes an effort to visualize the electron cloud that makes up the single O-H bond. You tell him that this will be complicated by the fact that the more you know about an electron's momentum, the less you know about its position, and vice versa. What principle are you describing?
Aufbau Principle
Principle of Electron Delocalization
Principle of Subatomic Probability
Hund's Rule
Heisenberg Uncertainty Principle
Heisenberg Uncertainty Principle
The Heisenberg Uncertainty Principle tells us exactly this. At the quantum level, the more you know about a body's momentum, the less you know about its position.
Example Question #2 : Other Principles Governing Electrons
Light waves are known to exist in a wave-particle duality. When objects approach the speed of light, this duality supposedly extends to other forms of matter. A 
This is a problem utilizing Einstein's equation:
Before we can solve for the energy, we need to convert the mass to kilograms.
Use this mass and the speed of light to solve for the energy.
Example Question #853 : Mcat Physical Sciences
Which of the following statements are true when there are two electrons in the 
One electron will be emitted
They must be occupying different orbitals
One electron will be absorbed
They must be oppositely charged
They must have opposite spins
They must have opposite spins
Electrons occupying the same exact subshell are essentially equivalent. The only thing that differs between them is spin. Because of the Pauli exclusion principle, the electrons must have opposite spins when occupying the same subshell in an atom. There is nothing to suggest any electrons are being absorbed or emitted.
Example Question #1 : Lewis Dot Structures
Isomers
Resonance structures
Isotopes
Epimers
Resonance structures
The answer is resonance structures. When different Lewis structures can be drawn for a single molecule, the molecule exists as a composite of these structures, which are called resonance structures.
Isotopes refers to multiple nuclear compositions for a single element, based on varying numbers of neutrons. Isomers are different configurations of a given molecular formula, based on geometry and orientation. Epimers are a specific class of isomers involving a single stereocenter.
Example Question #1 : Lewis Dot Structures
Diffusion can be defined as the net transfer of molecules down a gradient of differing concentrations. This is a passive and spontaneous process and relies on the random movement of molecules and Brownian motion. Diffusion is an important biological process, especially in the respiratory system where oxygen diffuses from alveoli, the basic unit of lung mechanics, to red blood cells in the capillaries.
Figure 1 depicts this process, showing an alveoli separated from neighboring cells by a capillary with red blood cells. The partial pressures of oxygen and carbon dioxide are given. One such equation used in determining gas exchange is Fick's law, given by:
ΔV = (Area/Thickness) · Dgas · (P1 – P2)
Where ΔV is flow rate and area and thickness refer to the permeable membrane through which the gas passes, in this case, the wall of the avlveoli. P1 and P2 refer to the partial pressures upstream and downstream, respectively. Further, Dgas, the diffusion constant of the gas, is defined as:
Dgas = Solubility / (Molecular Weight)^(1/2)
How many total valence electrons does carbon dioxide contain?
24
22
10
16
16
This is a straightforward question that has little to do with the passage, but it's a concept almost certainly to be seen on the MCAT.
Make sure to understand the concept of valence electrons and what that number may tell you about bonding properties. Questions like these are essentially "freebies," and should be answered without hesitation.
Example Question #1 : Lewis Dot Structures
How many valence electrons do boron and nitrogen have?
Boron has three valence electrons.
Nitrogen has five valence electrons.
Boron has five valence electrons.
Nitrogen has three valence electrons.
Boron has three valence electrons.
Nitrogen has four valence electrons.
Boron has seven valence electrons.
Nitrogen has eight valence electrons.
Boron has three valence electrons.
Nitrogen has five valence electrons.
To determine the number of electrons an atom has, you must look at which column the atom is in on the periodic table. Boron is in column 3A, so it has three valence electrons. Nitrogen is in column 5A, so it has five valence electrons.
You should be familiar with common elements, like nitrogen, without looking at the periodic table. This will save you time on the exam.
Example Question #1 : Vsepr Geometry
Which statement best describes VSEPR theory?
Covalent bonds are formed by overlapping valence electron shells
The repulsion between atoms helps determine the polarity of molecules
Molecular shapes are determined according to which orbitals are energetically accessible for bonding
The repulsion between electrons helps determine the geometry of covalent molecules
The repulsion between protons in an atom's nucleus determines its size
The repulsion between electrons helps determine the geometry of covalent molecules
The idea of VSEPR (valence shell electron pair repulsion) theory is that valence electron pairs repel each other, arranging themselves into positions that minimize their repulsions by maximizing the distance between them. The positions of these electron pairs then determine the overall geometry of the molecule. Molecular geometry is thus determined by the arrangement of electrons and nuclei such that the electrons are as far from one another as possible, while remaining as close to the positively charged nucleus as possible.
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