CHEM 120 Exam Predictor | High-Yield QUESTIONs,
Correct Answers & Detailed Rationales 2026/2027
QUESTION 1
What is the mass percentage of hydrogen in ammonium sulfate,
(𝑁𝐻4 )2 𝑆𝑂4 ? (Molar masses: N = 14.01 g/mol, H = 1.008 g/mol, S =
32.06 g/mol, O = 16.00 g/mol)
• A. 3.19%
• B. 6.11%
• C. 8.07%
• D. 12.1%
Correct Answer: B. 6.11%
Detailed Rationale: The molar mass of (𝑁𝐻4 )2 𝑆𝑂4 is (2 × 14.01) +
(8 × 1.008) + 32.06 + (4 × 16.00) = 28.02 + 8.064 + 32.06 +
64.00 = 132.14 g/mol. The total mass of hydrogen is 8 × 1.008 =
8.064 g/mol. The mass percentage is (8.064/132.14) × 100% =
6.102%, which rounds to 6.11%.
QUESTION 2
A sample of an unknown gas occupies 4.50 L at 310 K and 1.20 atm. If
the mass of this gas sample is 9.35 g, what is the molar mass of the
gas? (𝑅 = 0.08206 L$\cdot𝑎𝑡𝑚/𝑚𝑜𝑙\cdot$K)
• A. 32.0 g/mol
• B. 41.6 g/mol
, • C. 44.0 g/mol
• D. 58.1 g/mol
Correct Answer: C. 44.0 g/mol
𝑃𝑉
Detailed Rationale: Use the ideal gas law to find moles (𝑛): 𝑛 = =
𝑅𝑇
1.20 atm×4.50 L
= 0.2123 mol. Molar mass (ℳ) is mass divided
0.08206 L⋅atm/mol⋅K×310 K
9.35 g
by moles: ℳ = = 44.0 g/mol.
0.2123 mol
QUESTION 3
When 1.00 g of liquid ethanol (𝐶𝐻3 𝐶𝐻2 𝑂𝐻) undergoes complete
combustion, it releases 29.7 kJ of heat. What is the standard enthalpy
of combustion (Δ𝐻comb) of ethanol in kJ/mol? (Molar mass of ethanol =
46.07 g/mol)
• A. −644 kJ/mol
• B. −1,368 kJ/mol
• C. +1,368 kJ/mol
• D. −2,970 kJ/mol
Correct Answer: B. −1,368 kJ/mol
Detailed Rationale: Calculate moles of ethanol burned: 1.00 g/
46.07 g/mol = 0.02171 mol. The heat released per mole is Δ𝐻 =
−29.7 kJ
= −1,368 kJ/mol. Since heat is released, the enthalpy
0.02171 mol
change is negative.
QUESTION 4
,Which set of four quantum numbers (𝑛, 𝑙, 𝑚𝑙 , 𝑚𝑠 ) describes an electron
in a 4𝑝 orbital?
• A. 4,0,0, +1/2
• B. 4,1, −1, −1/2
• C. 3,1,0, +1/2
• D. 4,2,1, +1/2
Correct Answer: B. 4,1, −1, −1/2
Detailed Rationale: The principal quantum number 𝑛 = 4 indicates the
4th shell. A 𝑝 subshell corresponds to an angular momentum quantum
number 𝑙 = 1. The magnetic quantum number 𝑚𝑙 can range from −𝑙 to
+𝑙 (so −1,0, +1), and the spin quantum number 𝑚𝑠 can be +1/2 or
−1/2. Option B fits these criteria.
QUESTION 5
Which of the following atoms has the largest atomic radius?
• A. Fluorine (𝐹)
• B. Chlorine (𝐶𝑙)
• C. Bromine (𝐵𝑟)
• D. Iodine (𝐼)
Correct Answer: D. Iodine (𝐼)
Detailed Rationale: Atomic radius decreases across a period from left to
right and increases down a group in the periodic table. Iodine is located
lower down in Group 17 than fluorine, chlorine, or bromine, giving it
, the largest principal quantum level and atomic radius among the
choices.
QUESTION 6
What is the formal charge on the central nitrogen atom in the nitrate
ion (𝑁𝑂3− )?
• A. −1
• B. 0
• C. +1
• D. +2
Correct Answer: C. +1
Detailed Rationale: Formal charge = Valence electrons −
1
Non-bonding electrons − (Bonding electrons). In a standard
2
resonance structure of 𝑁𝑂3− ,
nitrogen has 5 valence electrons, zero
non-bonding electrons, and 8 bonding electrons (4 bonds).
Formal charge = 5 − 0 − 4 = +1.
QUESTION 7
According to VSEPR theory, what is the molecular geometry of the
sulfur tetrafluoride (𝑆𝐹4 ) molecule?
• A. Tetrahedral
• B. Square planar
• C. See-saw
• D. Trigonal bipyramidal
Correct Answer: C. See-saw
Correct Answers & Detailed Rationales 2026/2027
QUESTION 1
What is the mass percentage of hydrogen in ammonium sulfate,
(𝑁𝐻4 )2 𝑆𝑂4 ? (Molar masses: N = 14.01 g/mol, H = 1.008 g/mol, S =
32.06 g/mol, O = 16.00 g/mol)
• A. 3.19%
• B. 6.11%
• C. 8.07%
• D. 12.1%
Correct Answer: B. 6.11%
Detailed Rationale: The molar mass of (𝑁𝐻4 )2 𝑆𝑂4 is (2 × 14.01) +
(8 × 1.008) + 32.06 + (4 × 16.00) = 28.02 + 8.064 + 32.06 +
64.00 = 132.14 g/mol. The total mass of hydrogen is 8 × 1.008 =
8.064 g/mol. The mass percentage is (8.064/132.14) × 100% =
6.102%, which rounds to 6.11%.
QUESTION 2
A sample of an unknown gas occupies 4.50 L at 310 K and 1.20 atm. If
the mass of this gas sample is 9.35 g, what is the molar mass of the
gas? (𝑅 = 0.08206 L$\cdot𝑎𝑡𝑚/𝑚𝑜𝑙\cdot$K)
• A. 32.0 g/mol
• B. 41.6 g/mol
, • C. 44.0 g/mol
• D. 58.1 g/mol
Correct Answer: C. 44.0 g/mol
𝑃𝑉
Detailed Rationale: Use the ideal gas law to find moles (𝑛): 𝑛 = =
𝑅𝑇
1.20 atm×4.50 L
= 0.2123 mol. Molar mass (ℳ) is mass divided
0.08206 L⋅atm/mol⋅K×310 K
9.35 g
by moles: ℳ = = 44.0 g/mol.
0.2123 mol
QUESTION 3
When 1.00 g of liquid ethanol (𝐶𝐻3 𝐶𝐻2 𝑂𝐻) undergoes complete
combustion, it releases 29.7 kJ of heat. What is the standard enthalpy
of combustion (Δ𝐻comb) of ethanol in kJ/mol? (Molar mass of ethanol =
46.07 g/mol)
• A. −644 kJ/mol
• B. −1,368 kJ/mol
• C. +1,368 kJ/mol
• D. −2,970 kJ/mol
Correct Answer: B. −1,368 kJ/mol
Detailed Rationale: Calculate moles of ethanol burned: 1.00 g/
46.07 g/mol = 0.02171 mol. The heat released per mole is Δ𝐻 =
−29.7 kJ
= −1,368 kJ/mol. Since heat is released, the enthalpy
0.02171 mol
change is negative.
QUESTION 4
,Which set of four quantum numbers (𝑛, 𝑙, 𝑚𝑙 , 𝑚𝑠 ) describes an electron
in a 4𝑝 orbital?
• A. 4,0,0, +1/2
• B. 4,1, −1, −1/2
• C. 3,1,0, +1/2
• D. 4,2,1, +1/2
Correct Answer: B. 4,1, −1, −1/2
Detailed Rationale: The principal quantum number 𝑛 = 4 indicates the
4th shell. A 𝑝 subshell corresponds to an angular momentum quantum
number 𝑙 = 1. The magnetic quantum number 𝑚𝑙 can range from −𝑙 to
+𝑙 (so −1,0, +1), and the spin quantum number 𝑚𝑠 can be +1/2 or
−1/2. Option B fits these criteria.
QUESTION 5
Which of the following atoms has the largest atomic radius?
• A. Fluorine (𝐹)
• B. Chlorine (𝐶𝑙)
• C. Bromine (𝐵𝑟)
• D. Iodine (𝐼)
Correct Answer: D. Iodine (𝐼)
Detailed Rationale: Atomic radius decreases across a period from left to
right and increases down a group in the periodic table. Iodine is located
lower down in Group 17 than fluorine, chlorine, or bromine, giving it
, the largest principal quantum level and atomic radius among the
choices.
QUESTION 6
What is the formal charge on the central nitrogen atom in the nitrate
ion (𝑁𝑂3− )?
• A. −1
• B. 0
• C. +1
• D. +2
Correct Answer: C. +1
Detailed Rationale: Formal charge = Valence electrons −
1
Non-bonding electrons − (Bonding electrons). In a standard
2
resonance structure of 𝑁𝑂3− ,
nitrogen has 5 valence electrons, zero
non-bonding electrons, and 8 bonding electrons (4 bonds).
Formal charge = 5 − 0 − 4 = +1.
QUESTION 7
According to VSEPR theory, what is the molecular geometry of the
sulfur tetrafluoride (𝑆𝐹4 ) molecule?
• A. Tetrahedral
• B. Square planar
• C. See-saw
• D. Trigonal bipyramidal
Correct Answer: C. See-saw
