MCAT General Chemistry
Questions and Answers 100%
Correct
Which of the following increases with increasing atomic number within a family on
the periodic table?
A) electronegativity
B) electron affinity
C) atomic radius
D) ionization energy - ANSWER - C) atomic radius
Only atomic radius increases going down a column. Size of an atom is determined
not only by effective nuclear charge, but also by number of electron shells. Moving
down a column, the number of electron shells increases. Because the additional
shells require additional space, atomic radius increases as number of electron shells
increases.
Which of the following most likely represents the correct order of ion size from
greatest to smallest?
A) O2-, F-, Na+, Mg2+
B) Mg2+, Na+, F-, O2-
C) Na+, Mg2+, O2-, F-
D) Mg2+, Na+, O2-, F- - ANSWER - A) O2-, F-, Na+, Mg2+
This is an isoelectronic series, which means that the number of electrons on each
ion is the same. In an isoelectronic series, the nuclear charge increases with
increasing atomic number and draws the electrons inward with greater force. The
ion with the fewest protons produces the weakest attractive force on the ions, so it
has the largest size. Oxygen has the smallest atomic number, so it has the fewest
protons, and therefore will have the greatest ion size.
Which of the following best explains why sulfur can make more bonds than oxygen?
,A) sulfur is more electronegative than oxygen
B) oxygen is more electronegative than sulfur
C) sulfur has 3d orbitals not available to oxygen
D) sulfur has fewer valence electrons - ANSWER - C) sulfur has 3d orbitals not
available to oxygen
Only elements in the third and higher periods of the periodic table can form more
than 4 bonds. Second period elements, including oxygen, have four valence orbitals
(one 2s and three 2p) with which to form bonds. By contrast, third period elements
like sulfur can form bonds with not only one 3s and three 3p, but also five 3d
orbitals. Remember that the second quantum number can have any integer value
from 0 to n-1, so when n = 3, three subshells (s, p, and d) are available.
Which of the following species has an unpaired electron in its ground-state
electronic configuration?
A) Ne
B) Ca+
C) Na+
D) O2- - ANSWER - B) Ca+
Atoms and ions with electron configurations identical to those of noble gases do not
have any unpaired electrons in their ground state. Ne, Na+, and O2- all have the
same electron configuration, [Ne], and therefore have no unpaired electrons in their
ground state. Ca+, by contrast, has a ground state configuration of [Ar]4s1, and has
one unpaired electron in its 4s subshell.
What is the electron configuration of chromium?
A) [Ar] 3d6
B) [Ar] 4s13d5
C) [Ar] 4s23d3
D) [Ar] 4s24d4 - ANSWER - B) [Ar] 4s13d5
Which of the following molecules has the greatest dipole moment?
A) H2
,B) O2
C) HF
D) HBr - ANSWER - C) HF
The dipole moment will be greatest for the atoms with the greatest difference in
electronegativity. Based upon periodic trends, H and F will have the greatest
difference in electronegativity, and therefore the greatest dipole moment.
A natural sample of carbon contains 99% of C-12. How many moles of C-12 are
likely to be found in a 48.5 gram sample of carbon obtained from nature?
A) 1
B) 4
C) 12
D) 49.5 - ANSWER - B) 4
Solving this problem does not require complicated calculations. Simply assume that
100% of the sample is 12-C. The molecular weight of 12-C is 12 g/mol. 48.5 g x 12
g/mol = ~4 mol.
I-131 is a radionuclide used in the treatment of thyroid carcinomas. If a sample's
activity dropped from 100 mCi to 12.5 mCi in 12 days, what is the half-life of I-131?
A) 1.5 days
B) 3.0 days
C) 4.0 days
D) 6.0 days - ANSWER - C) 4.0 days
Even without pre-existing knowledge of the electron configuration of Cr, strongest
answer could have been identified by eliminating improbable choices. Choice C can
be eliminated because it has the wrong number of electrons. Choice D can be
eliminated because Cr ground state electrons exist in the 3d rather than 4d
subshell. Given Hund's rule, which states that the most stable arrangement of
electrons is the one with the most unpaired electrons, choice B might have seemed
more likely to be the best answer.
, In reference to the photoelectric effect, which of the following will increase the
kinetic energy of a photoelectron?
A) increasing the work function
B) increasing the frequency of the incident light
C) increasing the number of photons in the incident light
D) increasing the mass of photons in the incident light - ANSWER - B) increasing the
frequency of the incident light
KE = hf - phi, in which h is Planck's constant, f is the frequency of incident light, and
phi is the work function of the metal. In other words, only the frequency of the
incident light and the work function of the metal affect the KE of ejected electrons,
so C and D can be eliminated. Note that photons are conventionally held to be
massless, so this eliminates D as well. The work function is the minimum energy
required to eject an electron from a solid state into a surrounding vacuum.
Increasing the work function decreases the kinetic energy of the ejected electron.
Only increasing frequency increases KE.
When an electron moves from a 2p to a 3s orbital, the atom containing that
electron:
A) becomes an new isotope
B) becomes a new element
C) absorbs energy
D) releases energy - ANSWER - C) absorbs energy
3s orbitals are at higher energy than 2p orbitals. For an electron to move from 2p to
3s, the atom containing that electron must absorb energy. In order to become a new
isotope, the atom must gain or lose a neutron. In order to become a new element,
the atom must gain or lose a proton. The atom would release energy if the electron
moved to a lower energy orbital.
Aluminum only has one oxidation state, while chromium has several. Which of the
following is the best explanation for this difference?
A) electrons in the d orbitals of Cr may or may not be used to form bonds
B) electrons in the p orbitals of Cr may or may not be used to form bonds
C) electrons in the d orbitals of Al may or may not be used to form bonds
