CHEM 103 | CHEM103 Final Exam: General
Chemistry Updated and Latest Questions and
Correct Answers with Rationale - Portage Learning
1. What is the correct ground-state electron configuration for a neutral Chromium atom
(Z=24)?
A. [Ar] 4s2 3d4
B. [Ar] 3d6
C. [Ar] 4s2 3d5
D. [Ar] 4s1 3d5
Correct Answer: D
Expert Explanation: Chromium is a notable exception to the Aufbau principle because a
half-filled d-subshell offers increased stability. Instead of the expected 4s2 3d4
configuration, one electron from the 4s orbital moves to the 3d orbital. This resulting [Ar]
4s1 3d5 arrangement minimizes electron repulsion and maximizes exchange energy.
Understanding these exceptions is vital for predicting the magnetic properties and
reactivity of transition metals. This specific electronic structure illustrates how atoms
prioritize lower overall energy states over simple filling rules.
2. An element has two naturally occurring isotopes: Isotope A (35.0 amu) at 75% abundance
and Isotope B (37.0 amu) at 25% abundance. What is its average atomic mass?
A. 35.5 amu
B. 36.0 amu
C. 36.5 amu
D. 35.8 amu
Correct Answer: A
Expert Explanation: The average atomic mass is determined by calculating the weighted
average of all naturally occurring isotopes. You multiply the mass of each isotope by its
fractional abundance and sum the results together. For this problem, (35.0 * 0.75) + (37.0 *
0.25) equals 26.25 + 9.25, which totals 35.5 amu. This value represents the mass of a
typical sample of the element as found in nature. Such calculations are fundamental for
stoichiometry and relate microscopic atomic masses to macroscopic laboratory
measurements.
3. Which of the following sets of quantum numbers (n, l, ml, ms) is not allowed for an
electron in an atom?
A. 3, 2, 0, +1/2
,B. 2, 1, -1, -1/2
C. 2, 2, 1, -1/2
D. 1, 0, 0, +1/2
Correct Answer: C
Expert Explanation: Quantum mechanics dictates specific rules for the allowed values of
quantum numbers to describe electron states. The principal quantum number n must be a
positive integer, while l can range from 0 to n-1. In option D, if n=2, the maximum value for
l is 1, making l=2 invalid. This means a 2d orbital does not exist in a standard atomic
structure. Mastery of these rules is essential for understanding the shell structure and
periodic trends of elements.
4. How many grams of water are produced when 4.0 grams of H2 react with 32.0 grams of
O2? (2 H2 + O2 -> 2 H2O)
A. 18.0 g
B. 72.0 g
C. 9.0 g
D. 36.0 g
Correct Answer: D
Expert Explanation: To solve this stoichiometry problem, we first determine the moles of
each reactant using their molar masses. Four grams of H2 is approximately 2 moles, and 32
grams of O2 is exactly 1 mole. According to the balanced equation, 2 moles of H2 react
perfectly with 1 mole of O2 to produce 2 moles of water. Since 2 moles of water weigh 36.0
grams, this is the maximum theoretical yield. This problem demonstrates the law of
conservation of mass and the importance of stoichiometric ratios.
5. A hydrocarbon is found to be 80% Carbon and 20% Hydrogen by mass. What is its empirical
formula?
A. CH2
B. CH4
C. C2H5
D. CH3
Correct Answer: D
Expert Explanation: Finding an empirical formula requires converting mass percentages
into molar ratios of the elements involved. In a 100g sample, there are 80g of C (6.67
moles) and 20g of H (20 moles). Dividing both mole counts by the smallest value (6.67)
gives a ratio of approximately 1 Carbon to 3 Hydrogens. Therefore, the simplest whole-
, number ratio for this compound is CH3. This process is a key step in identifying unknown
chemical substances in analytical chemistry.
6. If a gas occupies 2.0 L at 300 K and 1.0 atm, what will be its volume if the temperature is
increased to 600 K at constant pressure?
A. 4.0 L
B. 1.0 L
C. 2.0 L
D. 8.0 L
Correct Answer: A
Expert Explanation: This problem utilizes Charles’s Law, which states that volume is
directly proportional to absolute temperature at constant pressure. When the Kelvin
temperature doubles from 300 K to 600 K, the volume must also double to maintain the
relationship. Thus, the initial 2.0 L volume increases to 4.0 L under these specific
conditions. It is important to always use the Kelvin scale for gas law calculations to ensure
mathematical accuracy. This principle explains why balloons expand when heated and
contract when cooled.
7. A mixture of gases contains 0.5 mol N2, 0.2 mol O2, and 0.3 mol He. If the total pressure is
2.0 atm, what is the partial pressure of O2?
A. 0.2 atm
B. 1.0 atm
C. 0.5 atm
D. 0.4 atm
Correct Answer: D
Expert Explanation: Dalton’s Law of Partial Pressures states that the pressure of an
individual gas in a mixture is proportional to its mole fraction. The total number of moles in
this mixture is 1.0, making the mole fraction of O2 equal to 0.2. Multiplying the mole
fraction of 0.2 by the total pressure of 2.0 atm yields a partial pressure of 0.4 atm. This
concept is vital for understanding respiratory physiology and industrial gas mixing. Each
gas behaves independently of the others within the container’s volume.
8. How many milliliters of 12.0 M HCl are needed to prepare 500 mL of a 2.0 M HCl solution?
A. 250.0 mL
B. 41.7 mL
C. 166.7 mL
D. 83.3 mL
Chemistry Updated and Latest Questions and
Correct Answers with Rationale - Portage Learning
1. What is the correct ground-state electron configuration for a neutral Chromium atom
(Z=24)?
A. [Ar] 4s2 3d4
B. [Ar] 3d6
C. [Ar] 4s2 3d5
D. [Ar] 4s1 3d5
Correct Answer: D
Expert Explanation: Chromium is a notable exception to the Aufbau principle because a
half-filled d-subshell offers increased stability. Instead of the expected 4s2 3d4
configuration, one electron from the 4s orbital moves to the 3d orbital. This resulting [Ar]
4s1 3d5 arrangement minimizes electron repulsion and maximizes exchange energy.
Understanding these exceptions is vital for predicting the magnetic properties and
reactivity of transition metals. This specific electronic structure illustrates how atoms
prioritize lower overall energy states over simple filling rules.
2. An element has two naturally occurring isotopes: Isotope A (35.0 amu) at 75% abundance
and Isotope B (37.0 amu) at 25% abundance. What is its average atomic mass?
A. 35.5 amu
B. 36.0 amu
C. 36.5 amu
D. 35.8 amu
Correct Answer: A
Expert Explanation: The average atomic mass is determined by calculating the weighted
average of all naturally occurring isotopes. You multiply the mass of each isotope by its
fractional abundance and sum the results together. For this problem, (35.0 * 0.75) + (37.0 *
0.25) equals 26.25 + 9.25, which totals 35.5 amu. This value represents the mass of a
typical sample of the element as found in nature. Such calculations are fundamental for
stoichiometry and relate microscopic atomic masses to macroscopic laboratory
measurements.
3. Which of the following sets of quantum numbers (n, l, ml, ms) is not allowed for an
electron in an atom?
A. 3, 2, 0, +1/2
,B. 2, 1, -1, -1/2
C. 2, 2, 1, -1/2
D. 1, 0, 0, +1/2
Correct Answer: C
Expert Explanation: Quantum mechanics dictates specific rules for the allowed values of
quantum numbers to describe electron states. The principal quantum number n must be a
positive integer, while l can range from 0 to n-1. In option D, if n=2, the maximum value for
l is 1, making l=2 invalid. This means a 2d orbital does not exist in a standard atomic
structure. Mastery of these rules is essential for understanding the shell structure and
periodic trends of elements.
4. How many grams of water are produced when 4.0 grams of H2 react with 32.0 grams of
O2? (2 H2 + O2 -> 2 H2O)
A. 18.0 g
B. 72.0 g
C. 9.0 g
D. 36.0 g
Correct Answer: D
Expert Explanation: To solve this stoichiometry problem, we first determine the moles of
each reactant using their molar masses. Four grams of H2 is approximately 2 moles, and 32
grams of O2 is exactly 1 mole. According to the balanced equation, 2 moles of H2 react
perfectly with 1 mole of O2 to produce 2 moles of water. Since 2 moles of water weigh 36.0
grams, this is the maximum theoretical yield. This problem demonstrates the law of
conservation of mass and the importance of stoichiometric ratios.
5. A hydrocarbon is found to be 80% Carbon and 20% Hydrogen by mass. What is its empirical
formula?
A. CH2
B. CH4
C. C2H5
D. CH3
Correct Answer: D
Expert Explanation: Finding an empirical formula requires converting mass percentages
into molar ratios of the elements involved. In a 100g sample, there are 80g of C (6.67
moles) and 20g of H (20 moles). Dividing both mole counts by the smallest value (6.67)
gives a ratio of approximately 1 Carbon to 3 Hydrogens. Therefore, the simplest whole-
, number ratio for this compound is CH3. This process is a key step in identifying unknown
chemical substances in analytical chemistry.
6. If a gas occupies 2.0 L at 300 K and 1.0 atm, what will be its volume if the temperature is
increased to 600 K at constant pressure?
A. 4.0 L
B. 1.0 L
C. 2.0 L
D. 8.0 L
Correct Answer: A
Expert Explanation: This problem utilizes Charles’s Law, which states that volume is
directly proportional to absolute temperature at constant pressure. When the Kelvin
temperature doubles from 300 K to 600 K, the volume must also double to maintain the
relationship. Thus, the initial 2.0 L volume increases to 4.0 L under these specific
conditions. It is important to always use the Kelvin scale for gas law calculations to ensure
mathematical accuracy. This principle explains why balloons expand when heated and
contract when cooled.
7. A mixture of gases contains 0.5 mol N2, 0.2 mol O2, and 0.3 mol He. If the total pressure is
2.0 atm, what is the partial pressure of O2?
A. 0.2 atm
B. 1.0 atm
C. 0.5 atm
D. 0.4 atm
Correct Answer: D
Expert Explanation: Dalton’s Law of Partial Pressures states that the pressure of an
individual gas in a mixture is proportional to its mole fraction. The total number of moles in
this mixture is 1.0, making the mole fraction of O2 equal to 0.2. Multiplying the mole
fraction of 0.2 by the total pressure of 2.0 atm yields a partial pressure of 0.4 atm. This
concept is vital for understanding respiratory physiology and industrial gas mixing. Each
gas behaves independently of the others within the container’s volume.
8. How many milliliters of 12.0 M HCl are needed to prepare 500 mL of a 2.0 M HCl solution?
A. 250.0 mL
B. 41.7 mL
C. 166.7 mL
D. 83.3 mL
