ACS General Chemistry Exam Prep 2026 |
Comprehensive Practice Test, Study Guide &
Detailed Answer Explanations for Chemistry
Success
• This guide offers 200 full-length practice testing across all major ACS General
Chemistry topics with detailed EXPERT RATIONALE for every correct answer
to reinforce conceptual understanding.
• Best used in timed sets of 25–30 questions, reviewing EXPERT RATIONALE
immediately after each set to identify and target weak areas before moving
on.
Question 1: Which subatomic particle has a negligible mass compared to
protons and neutrons?
A. Proton
B. Neutron
C. Electron ✓ (Correct Answer)
D. Nucleon
E. Positron
EXPERT RATIONALE: Electrons have a mass of approximately 1/1836 that of a
proton, making their mass negligible in atomic mass calculations. Protons and
neutrons contribute almost all of an atom's mass.
Question 2: What is the electron configuration of a neutral chlorine atom (Z =
17)?
A. 1s² 2s² 2p⁶ 3s² 3p⁴
B. 1s² 2s² 2p⁶ 3s² 3p⁵ ✓ (Correct Answer)
C. 1s² 2s² 2p⁶ 3s² 3p⁶
D. 1s² 2s² 2p⁵ 3s² 3p⁶
,E. 1s² 2s² 2p⁶ 3s¹ 3p⁶
EXPERT RATIONALE: Chlorine has 17 electrons. Filling orbitals in order of
increasing energy gives 1s² 2s² 2p⁶ 3s² 3p⁵, totaling 17 electrons, with the last
electron entering the 3p subshell.
Question 3: Which of the following elements has the largest atomic radius?
A. Fluorine
B. Chlorine
C. Bromine
D. Iodine
E. Astatine ✓ (Correct Answer)
EXPERT RATIONALE: Atomic radius increases down a group due to the addition of
electron shells. Astatine, being lowest in the halogen group, has the largest atomic
radius despite increased nuclear charge.
Question 4: How many moles of oxygen atoms are present in 2.0 moles of
Ca(NO₃)₂?
A. 2 moles
B. 4 moles
C. 6 moles
D. 12 moles ✓ (Correct Answer)
E. 18 moles
EXPERT RATIONALE: Each Ca(NO₃)₂ formula unit contains 6 oxygen atoms (2
nitrate groups × 3 oxygens each). Multiplying by 2.0 moles gives 12 moles of oxygen
atoms.
,Question 5: Which quantum number describes the shape of an orbital?
A. Principal quantum number (n)
B. Angular momentum quantum number (l) ✓ (Correct Answer)
C. Magnetic quantum number (m_l)
D. Spin quantum number (m_s)
E. Effective nuclear charge
EXPERT RATIONALE: The angular momentum quantum number (l) determines the
shape of an orbital (s, p, d, f), while n determines size/energy level and m_l
determines orientation.
Question 6: A sample of an unknown compound contains 40.0% carbon, 6.7%
hydrogen, and 53.3% oxygen by mass. What is its empirical formula?
A. CH₂O ✓ (Correct Answer)
B. C₂H₄O₂
C. CHO
D. C₂H₆O
E. CH₃O
EXPERT RATIONALE: Converting percentages to moles (C: 40.0/12 = 3.33, H: 6.7/1 =
6.7, O: 53.3/16 = 3.33) and dividing by the smallest value (3.33) gives a ratio of 1:2:1,
yielding the empirical formula CH₂O.
Question 7: Which of the following best describes an isotope?
A. Atoms with the same number of protons but different numbers of neutrons ✓
(Correct Answer)
B. Atoms with the same number of neutrons but different numbers of protons
C. Atoms with the same mass number but different atomic numbers
, D. Atoms with different numbers of electrons only
E. Molecules with the same molecular formula but different structures
EXPERT RATIONALE: Isotopes are atoms of the same element (same proton
number, defining identity) that differ in neutron number, resulting in different mass
numbers.
Question 8: What is the mass, in grams, of 0.500 mol of glucose (C₆H₁₂O₆,
molar mass = 180.16 g/mol)?
A. 36.0 g
B. 45.0 g
C. 90.1 g ✓ (Correct Answer)
D. 180.2 g
E. 360.3 g
EXPERT RATIONALE: Mass = moles × molar mass = 0.500 mol × 180.16 g/mol = 90.1
g.
Question 9: Which trend correctly describes ionization energy across a period
(left to right)?
A. Ionization energy decreases steadily
B. Ionization energy generally increases ✓ (Correct Answer)
C. Ionization energy remains constant
D. Ionization energy increases then sharply drops at every element
E. Ionization energy is unrelated to nuclear charge
EXPERT RATIONALE: Across a period, increasing nuclear charge pulls electrons
closer and more tightly, generally increasing the energy required to remove an
electron, despite some minor exceptions.
Comprehensive Practice Test, Study Guide &
Detailed Answer Explanations for Chemistry
Success
• This guide offers 200 full-length practice testing across all major ACS General
Chemistry topics with detailed EXPERT RATIONALE for every correct answer
to reinforce conceptual understanding.
• Best used in timed sets of 25–30 questions, reviewing EXPERT RATIONALE
immediately after each set to identify and target weak areas before moving
on.
Question 1: Which subatomic particle has a negligible mass compared to
protons and neutrons?
A. Proton
B. Neutron
C. Electron ✓ (Correct Answer)
D. Nucleon
E. Positron
EXPERT RATIONALE: Electrons have a mass of approximately 1/1836 that of a
proton, making their mass negligible in atomic mass calculations. Protons and
neutrons contribute almost all of an atom's mass.
Question 2: What is the electron configuration of a neutral chlorine atom (Z =
17)?
A. 1s² 2s² 2p⁶ 3s² 3p⁴
B. 1s² 2s² 2p⁶ 3s² 3p⁵ ✓ (Correct Answer)
C. 1s² 2s² 2p⁶ 3s² 3p⁶
D. 1s² 2s² 2p⁵ 3s² 3p⁶
,E. 1s² 2s² 2p⁶ 3s¹ 3p⁶
EXPERT RATIONALE: Chlorine has 17 electrons. Filling orbitals in order of
increasing energy gives 1s² 2s² 2p⁶ 3s² 3p⁵, totaling 17 electrons, with the last
electron entering the 3p subshell.
Question 3: Which of the following elements has the largest atomic radius?
A. Fluorine
B. Chlorine
C. Bromine
D. Iodine
E. Astatine ✓ (Correct Answer)
EXPERT RATIONALE: Atomic radius increases down a group due to the addition of
electron shells. Astatine, being lowest in the halogen group, has the largest atomic
radius despite increased nuclear charge.
Question 4: How many moles of oxygen atoms are present in 2.0 moles of
Ca(NO₃)₂?
A. 2 moles
B. 4 moles
C. 6 moles
D. 12 moles ✓ (Correct Answer)
E. 18 moles
EXPERT RATIONALE: Each Ca(NO₃)₂ formula unit contains 6 oxygen atoms (2
nitrate groups × 3 oxygens each). Multiplying by 2.0 moles gives 12 moles of oxygen
atoms.
,Question 5: Which quantum number describes the shape of an orbital?
A. Principal quantum number (n)
B. Angular momentum quantum number (l) ✓ (Correct Answer)
C. Magnetic quantum number (m_l)
D. Spin quantum number (m_s)
E. Effective nuclear charge
EXPERT RATIONALE: The angular momentum quantum number (l) determines the
shape of an orbital (s, p, d, f), while n determines size/energy level and m_l
determines orientation.
Question 6: A sample of an unknown compound contains 40.0% carbon, 6.7%
hydrogen, and 53.3% oxygen by mass. What is its empirical formula?
A. CH₂O ✓ (Correct Answer)
B. C₂H₄O₂
C. CHO
D. C₂H₆O
E. CH₃O
EXPERT RATIONALE: Converting percentages to moles (C: 40.0/12 = 3.33, H: 6.7/1 =
6.7, O: 53.3/16 = 3.33) and dividing by the smallest value (3.33) gives a ratio of 1:2:1,
yielding the empirical formula CH₂O.
Question 7: Which of the following best describes an isotope?
A. Atoms with the same number of protons but different numbers of neutrons ✓
(Correct Answer)
B. Atoms with the same number of neutrons but different numbers of protons
C. Atoms with the same mass number but different atomic numbers
, D. Atoms with different numbers of electrons only
E. Molecules with the same molecular formula but different structures
EXPERT RATIONALE: Isotopes are atoms of the same element (same proton
number, defining identity) that differ in neutron number, resulting in different mass
numbers.
Question 8: What is the mass, in grams, of 0.500 mol of glucose (C₆H₁₂O₆,
molar mass = 180.16 g/mol)?
A. 36.0 g
B. 45.0 g
C. 90.1 g ✓ (Correct Answer)
D. 180.2 g
E. 360.3 g
EXPERT RATIONALE: Mass = moles × molar mass = 0.500 mol × 180.16 g/mol = 90.1
g.
Question 9: Which trend correctly describes ionization energy across a period
(left to right)?
A. Ionization energy decreases steadily
B. Ionization energy generally increases ✓ (Correct Answer)
C. Ionization energy remains constant
D. Ionization energy increases then sharply drops at every element
E. Ionization energy is unrelated to nuclear charge
EXPERT RATIONALE: Across a period, increasing nuclear charge pulls electrons
closer and more tightly, generally increasing the energy required to remove an
electron, despite some minor exceptions.
