CHEM 121 FINAL EXAM Actual Exam 2026/2027 –
Portage Learning | Complete Exam-Style Questions with
Detailed Rationales | 100% Verified | Pass Guaranteed –
A+ Graded
Atomic Structure, Periodicity, and Chemical Foundations
Q1: A student is analyzing a sample of naturally occurring copper, which consists of 69.15%
Cu-63 (mass 62.93 amu) and 30.85% Cu-65 (mass 64.93 amu). What is the average atomic
mass of this copper sample?
A. 63.55 amu [CORRECT]
B. 63.93 amu
C. 64.50 amu
D. 65.10 amu
Correct Answer: A
Rationale: The best answer is 63.55 amu because it correctly calculates the weighted
average of the isotopes based on their natural abundances and isotopic masses.
Q2: Which of the following electron configurations represents a neutral atom in an excited
state?
A. 1s² 2s² 2p⁶ 3s² 3p⁶
B. 1s² 2s² 2p⁶ 3s¹ 3p¹ [CORRECT]
C. 1s² 2s² 2p⁶ 3s² 3p⁵
D. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Correct Answer: B
Rationale: This choice is correct because an electron has been promoted from the lower-
energy 3s orbital to the higher-energy 3p orbital, which defines an excited state.
Q3: A lab technician needs to identify an unknown element that has a larger atomic radius
than sulfur but a higher first ionization energy than phosphorus. Which element fits this
description?
A. Silicon
B. Chlorine
C. Oxygen
D. None of the above [CORRECT]
Correct Answer: D
Rationale: This matches the principle that atomic radius decreases and ionization energy
generally increases across a period, making it impossible for one element to be both larger
than sulfur and have a higher ionization energy than phosphorus in the same period.
Q4: How many protons, neutrons, and electrons are present in a single ion of ⁵⁶Fe³⁺?
A. 26 protons, 30 neutrons, 23 electrons [CORRECT]
,B. 26 protons, 30 neutrons, 26 electrons
C. 29 protons, 27 neutrons, 26 electrons
D. 26 protons, 26 neutrons, 23 electrons
Correct Answer: A
Rationale: The best answer is 26 protons, 30 neutrons, and 23 electrons because the atomic
number is 26, the mass number (56) minus protons gives 30 neutrons, and the 3+ charge
indicates a loss of three electrons.
Q5: A student weighs out 45.0 g of calcium carbonate (CaCO₃). How many moles of CaCO₃
does this sample contain? (Molar mass of CaCO₃ = 100.09 g/mol)
A. 0.225 mol
B. 0.450 mol [CORRECT]
C. 2.22 mol
D. 4.50 mol
Correct Answer: B
Rationale: This choice is correct because dividing the given mass (45.0 g) by the molar mass
(100.09 g/mol) yields approximately 0.450 moles.
Q6: What is the empirical formula of a compound that is 40.0% carbon, 6.7% hydrogen, and
53.3% oxygen by mass?
A. CH₂O [CORRECT]
B. C₂H₄O₂
C. C₃H₆O₃
D. CHO
Correct Answer: A
Rationale: The best answer is CH₂O because converting the percentages to moles and
dividing by the smallest mole value yields a 1:2:1 ratio of C:H:O.
Q7: If the empirical formula of a compound is CH₂O and its molar mass is 180.16 g/mol,
what is its molecular formula?
A. C₃H₆O₃
B. C₄H₈O₄
C. C₅H₁₀O₅
D. C₆H₁₂O₆ [CORRECT]
Correct Answer: D
Rationale: This aligns with the principle that the molecular formula is a whole-number
multiple of the empirical formula; dividing 180.16 by the empirical mass (~30 g/mol) gives a
multiplier of 6.
Q8: A chemistry student performs a calculation: (4.56 × 1.4) + 2.1. According to the rules of
significant figures, how should the final answer be reported?
A. 8.484
B. 8.48
C. 8.5 [CORRECT]
D. 8
Correct Answer: C
, Rationale: The best answer is 8.5 because the multiplication step (4.56 × 1.4) yields 6.4 (two
sig figs), and adding 2.1 (one decimal place) results in 8.5, which maintains the correct
precision.
Q9: Which of the following sets of quantum numbers (n, l, mₗ, mₛ) is not allowed for an
electron in an atom?
A. 3, 2, -1, +½
B. 2, 1, 0, -½
C. 3, 3, 1, +½ [CORRECT]
D. 4, 0, 0, -½
Correct Answer: C
Rationale: This choice is correct because the angular momentum quantum number (l) must
be less than the principal quantum number (n); here, l cannot equal 3 when n is 3.
Q10: Calculate the energy of a photon of light with a wavelength of 450 nm. (h = 6.626 ×
10⁻³⁴ J·s, c = 3.00 × 10⁸ m/s)
A. 2.21 × 10⁻¹⁹ J
B. 4.42 × 10⁻¹⁹ J [CORRECT]
C. 6.63 × 10⁻¹⁹ J
D. 8.84 × 10⁻¹⁹ J
Correct Answer: B
Rationale: This matches the principle that E = hc/λ; plugging in the constants and converting
450 nm to meters yields approximately 4.42 × 10⁻¹⁹ J.
Q11: A laboratory procedure requires 0.250 moles of sodium chloride. What mass of NaCl
should the student weigh out? (Molar mass NaCl = 58.44 g/mol)
A. 11.7 g
B. 14.6 g [CORRECT]
C. 23.4 g
D. 58.4 g
Correct Answer: B
Rationale: The best answer is 14.6 g because multiplying the required moles (0.250 mol) by
the molar mass (58.44 g/mol) gives 14.61 g, which rounds to 14.6 g.
Q12: Which element is a metalloid that is in the same group as carbon?
A. Boron
B. Silicon [CORRECT]
C. Germanium
D. Arsenic
Correct Answer: B
Rationale: This choice is correct because silicon is the metalloid located directly below
carbon in Group 14 of the periodic table.
Q13: A student dissolves 5.00 g of an unknown metal in acid and collects 0.077 moles of
hydrogen gas. If the reaction stoichiometry is 1 mole of metal to 1 mole of H₂, what is the
approximate molar mass of the metal?
A. 32.5 g/mol
Portage Learning | Complete Exam-Style Questions with
Detailed Rationales | 100% Verified | Pass Guaranteed –
A+ Graded
Atomic Structure, Periodicity, and Chemical Foundations
Q1: A student is analyzing a sample of naturally occurring copper, which consists of 69.15%
Cu-63 (mass 62.93 amu) and 30.85% Cu-65 (mass 64.93 amu). What is the average atomic
mass of this copper sample?
A. 63.55 amu [CORRECT]
B. 63.93 amu
C. 64.50 amu
D. 65.10 amu
Correct Answer: A
Rationale: The best answer is 63.55 amu because it correctly calculates the weighted
average of the isotopes based on their natural abundances and isotopic masses.
Q2: Which of the following electron configurations represents a neutral atom in an excited
state?
A. 1s² 2s² 2p⁶ 3s² 3p⁶
B. 1s² 2s² 2p⁶ 3s¹ 3p¹ [CORRECT]
C. 1s² 2s² 2p⁶ 3s² 3p⁵
D. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Correct Answer: B
Rationale: This choice is correct because an electron has been promoted from the lower-
energy 3s orbital to the higher-energy 3p orbital, which defines an excited state.
Q3: A lab technician needs to identify an unknown element that has a larger atomic radius
than sulfur but a higher first ionization energy than phosphorus. Which element fits this
description?
A. Silicon
B. Chlorine
C. Oxygen
D. None of the above [CORRECT]
Correct Answer: D
Rationale: This matches the principle that atomic radius decreases and ionization energy
generally increases across a period, making it impossible for one element to be both larger
than sulfur and have a higher ionization energy than phosphorus in the same period.
Q4: How many protons, neutrons, and electrons are present in a single ion of ⁵⁶Fe³⁺?
A. 26 protons, 30 neutrons, 23 electrons [CORRECT]
,B. 26 protons, 30 neutrons, 26 electrons
C. 29 protons, 27 neutrons, 26 electrons
D. 26 protons, 26 neutrons, 23 electrons
Correct Answer: A
Rationale: The best answer is 26 protons, 30 neutrons, and 23 electrons because the atomic
number is 26, the mass number (56) minus protons gives 30 neutrons, and the 3+ charge
indicates a loss of three electrons.
Q5: A student weighs out 45.0 g of calcium carbonate (CaCO₃). How many moles of CaCO₃
does this sample contain? (Molar mass of CaCO₃ = 100.09 g/mol)
A. 0.225 mol
B. 0.450 mol [CORRECT]
C. 2.22 mol
D. 4.50 mol
Correct Answer: B
Rationale: This choice is correct because dividing the given mass (45.0 g) by the molar mass
(100.09 g/mol) yields approximately 0.450 moles.
Q6: What is the empirical formula of a compound that is 40.0% carbon, 6.7% hydrogen, and
53.3% oxygen by mass?
A. CH₂O [CORRECT]
B. C₂H₄O₂
C. C₃H₆O₃
D. CHO
Correct Answer: A
Rationale: The best answer is CH₂O because converting the percentages to moles and
dividing by the smallest mole value yields a 1:2:1 ratio of C:H:O.
Q7: If the empirical formula of a compound is CH₂O and its molar mass is 180.16 g/mol,
what is its molecular formula?
A. C₃H₆O₃
B. C₄H₈O₄
C. C₅H₁₀O₅
D. C₆H₁₂O₆ [CORRECT]
Correct Answer: D
Rationale: This aligns with the principle that the molecular formula is a whole-number
multiple of the empirical formula; dividing 180.16 by the empirical mass (~30 g/mol) gives a
multiplier of 6.
Q8: A chemistry student performs a calculation: (4.56 × 1.4) + 2.1. According to the rules of
significant figures, how should the final answer be reported?
A. 8.484
B. 8.48
C. 8.5 [CORRECT]
D. 8
Correct Answer: C
, Rationale: The best answer is 8.5 because the multiplication step (4.56 × 1.4) yields 6.4 (two
sig figs), and adding 2.1 (one decimal place) results in 8.5, which maintains the correct
precision.
Q9: Which of the following sets of quantum numbers (n, l, mₗ, mₛ) is not allowed for an
electron in an atom?
A. 3, 2, -1, +½
B. 2, 1, 0, -½
C. 3, 3, 1, +½ [CORRECT]
D. 4, 0, 0, -½
Correct Answer: C
Rationale: This choice is correct because the angular momentum quantum number (l) must
be less than the principal quantum number (n); here, l cannot equal 3 when n is 3.
Q10: Calculate the energy of a photon of light with a wavelength of 450 nm. (h = 6.626 ×
10⁻³⁴ J·s, c = 3.00 × 10⁸ m/s)
A. 2.21 × 10⁻¹⁹ J
B. 4.42 × 10⁻¹⁹ J [CORRECT]
C. 6.63 × 10⁻¹⁹ J
D. 8.84 × 10⁻¹⁹ J
Correct Answer: B
Rationale: This matches the principle that E = hc/λ; plugging in the constants and converting
450 nm to meters yields approximately 4.42 × 10⁻¹⁹ J.
Q11: A laboratory procedure requires 0.250 moles of sodium chloride. What mass of NaCl
should the student weigh out? (Molar mass NaCl = 58.44 g/mol)
A. 11.7 g
B. 14.6 g [CORRECT]
C. 23.4 g
D. 58.4 g
Correct Answer: B
Rationale: The best answer is 14.6 g because multiplying the required moles (0.250 mol) by
the molar mass (58.44 g/mol) gives 14.61 g, which rounds to 14.6 g.
Q12: Which element is a metalloid that is in the same group as carbon?
A. Boron
B. Silicon [CORRECT]
C. Germanium
D. Arsenic
Correct Answer: B
Rationale: This choice is correct because silicon is the metalloid located directly below
carbon in Group 14 of the periodic table.
Q13: A student dissolves 5.00 g of an unknown metal in acid and collects 0.077 moles of
hydrogen gas. If the reaction stoichiometry is 1 mole of metal to 1 mole of H₂, what is the
approximate molar mass of the metal?
A. 32.5 g/mol
