CHEM 120 Comprehensive Practice Exam | Latest
QUESTIONs, Correct Answers & Detailed Rationales
(2026/2027)
QUESTION 1
What is the empirical formula of a compound that contains 40.0%
carbon, 6.7% hydrogen, and 53.3% oxygen by mass? (Molar masses: C
= 12.01 g/mol, H = 1.008 g/mol, O = 16.00 g/mol)
• A. 𝐶𝐻2 𝑂
• B. 𝐶𝐻𝑂
• C. 𝐶2 𝐻4 𝑂2
• D. 𝐶𝐻4 𝑂
Correct Answer: A. 𝐶𝐻2 𝑂
Detailed Rationale: Assume 100 g of the compound, giving 40.0 g C,
6.7 g H, and 53.3 g O. Convert to moles: moles C = 40.0/12.01 = 3.33
mol; moles H = 6.7/1.008 = 6.65 mol; moles O = 53.3/16.00 = 3.33
mol. Divide by the smallest number of moles (3.33) to get the simplest
mole ratio: C : H : O = 1: 2: 1, yielding the empirical formula 𝐶𝐻2 𝑂.
QUESTION 2
An element has two naturally occurring isotopes. Isotope 1 has a mass
of 68.925 amu and a fractional abundance of 0.6011. Isotope 2 has a
mass of 70.925 amu and a fractional abundance of 0.3989. What is the
average atomic mass of this element?
, • A. 69.72 amu
• B. 69.92 amu
• C. 70.12 amu
• D. 70.52 amu
Correct Answer: A. 69.72 amu
Detailed Rationale: The average atomic mass is calculated by
multiplying the mass of each isotope by its fractional abundance and
summing the results: (68.925 × 0.6011) + (70.925 × 0.3989) =
41.43 + 28.29 = 69.72 amu.
QUESTION 3
What is the chemical formula for iron(III) sulfate?
• A. 𝐹𝑒𝑆𝑂4
• B. 𝐹𝑒2 (𝑆𝑂4 )3
• C. 𝐹𝑒3 (𝑆𝑂4 )2
• D. 𝐹𝑒2 𝑆𝑂3
Correct Answer: B. 𝐹𝑒2 (𝑆𝑂4 )3
Detailed Rationale: Iron(III) contains the 𝐹𝑒 3+ cation, and sulfate is the
polyatomic anion 𝑆𝑂42− . To balance the electrical charges, two iron(III)
ions (2 × +3 = +6) combine with three sulfate ions (3 × −2 = −6),
resulting in the formula 𝐹𝑒2 (𝑆𝑂4 )3 .
QUESTION 4
What volume of 0.250 M 𝑁𝑎𝑂𝐻 solution is required to completely
neutralize 50.0 mL of 0.150 M 𝐻𝐶𝑙?
, • A. 20.0 mL
• B. 30.0 mL
• C. 50.0 mL
• D. 83.3 mL
Correct Answer: B. 30.0 mL
Detailed Rationale: The neutralization reaction is 𝐻𝐶𝑙 (𝑎𝑞 ) +
𝑁𝑎𝑂𝐻 (𝑎𝑞 ) → 𝑁𝑎𝐶𝑙 (𝑎𝑞 ) + 𝐻2 𝑂(𝑙), which has a 1: 1 stoichiometric
ratio. Calculate moles of 𝐻𝐶𝑙: 𝑀1 𝑉1 = 0.150 mol/L × 0.0500 L =
0.00750 moles. This requires 0.00750 moles of 𝑁𝑎𝑂𝐻. The volume of
𝑁𝑎𝑂𝐻 is calculated as Moles/Molarity = 0.00750 mol/0.250 mol/L =
0.0300 L, or 30.0 mL.
QUESTION 5
A sample of gas occupies 3.20 L at 25∘ C and 1.20 atm. How many moles
of gas are present in the sample? (𝑅 = 0.08206 L$\cdot𝑎𝑡𝑚/
𝑚𝑜𝑙\cdot$K)
• A. 0.128 mol
• B. 0.157 mol
• C. 1.28 mol
• D. 3.10 mol
Correct Answer: B. 0.157 mol
Detailed Rationale: Use the ideal gas law equation 𝑃𝑉 = 𝑛𝑅𝑇. Convert
temperature to Kelvin: 𝑇 = 25 + 273.15 = 298.15 K. Rearrange for
𝑃𝑉 1.20 atm×3.20 L
moles (𝑛): 𝑛 = = = 0.157 mol.
𝑅𝑇 0.08206 L⋅atm/mol⋅K×298.15 K
, QUESTION 6
When 2.50 g of a solid dissolves in 100.0 g of water in a calorimeter,
the temperature of the water drops from 22.5∘ C to 18.1∘ C. What can
be concluded about this dissolution process?
• A. It is exothermic, and Δ𝐻 is positive.
• B. It is endothermic, and Δ𝐻 is positive.
• C. It is exothermic, and Δ𝐻 is negative.
• D. It is endothermic, and Δ𝐻 is negative.
Correct Answer: B. It is endothermic, and Δ𝐻 is positive.
Detailed Rationale: A decrease in the temperature of the water
indicates that the dissolving process absorbed heat from its
surroundings, making it endothermic. For an endothermic process, the
system absorbs heat, meaning the change in enthalpy (Δ𝐻) is positive.
QUESTION 7
What is the ground-state electron configuration of a neutral chromium
atom (𝐶𝑟, atomic number 24)?
• A. [𝐴𝑟]4𝑠 2 3𝑑 4
• B. [𝐴𝑟]4𝑠1 3𝑑 5
• C. [𝐴𝑟]4𝑠 2 3𝑑 9
• D. [𝐴𝑟]4𝑠 0 3𝑑 6
Correct Answer: B. [𝐴𝑟]4𝑠1 3𝑑5
Detailed Rationale: Chromium is an exception to the standard filling
order due to the extra stability associated with half-filled 𝑑-subshells.
QUESTIONs, Correct Answers & Detailed Rationales
(2026/2027)
QUESTION 1
What is the empirical formula of a compound that contains 40.0%
carbon, 6.7% hydrogen, and 53.3% oxygen by mass? (Molar masses: C
= 12.01 g/mol, H = 1.008 g/mol, O = 16.00 g/mol)
• A. 𝐶𝐻2 𝑂
• B. 𝐶𝐻𝑂
• C. 𝐶2 𝐻4 𝑂2
• D. 𝐶𝐻4 𝑂
Correct Answer: A. 𝐶𝐻2 𝑂
Detailed Rationale: Assume 100 g of the compound, giving 40.0 g C,
6.7 g H, and 53.3 g O. Convert to moles: moles C = 40.0/12.01 = 3.33
mol; moles H = 6.7/1.008 = 6.65 mol; moles O = 53.3/16.00 = 3.33
mol. Divide by the smallest number of moles (3.33) to get the simplest
mole ratio: C : H : O = 1: 2: 1, yielding the empirical formula 𝐶𝐻2 𝑂.
QUESTION 2
An element has two naturally occurring isotopes. Isotope 1 has a mass
of 68.925 amu and a fractional abundance of 0.6011. Isotope 2 has a
mass of 70.925 amu and a fractional abundance of 0.3989. What is the
average atomic mass of this element?
, • A. 69.72 amu
• B. 69.92 amu
• C. 70.12 amu
• D. 70.52 amu
Correct Answer: A. 69.72 amu
Detailed Rationale: The average atomic mass is calculated by
multiplying the mass of each isotope by its fractional abundance and
summing the results: (68.925 × 0.6011) + (70.925 × 0.3989) =
41.43 + 28.29 = 69.72 amu.
QUESTION 3
What is the chemical formula for iron(III) sulfate?
• A. 𝐹𝑒𝑆𝑂4
• B. 𝐹𝑒2 (𝑆𝑂4 )3
• C. 𝐹𝑒3 (𝑆𝑂4 )2
• D. 𝐹𝑒2 𝑆𝑂3
Correct Answer: B. 𝐹𝑒2 (𝑆𝑂4 )3
Detailed Rationale: Iron(III) contains the 𝐹𝑒 3+ cation, and sulfate is the
polyatomic anion 𝑆𝑂42− . To balance the electrical charges, two iron(III)
ions (2 × +3 = +6) combine with three sulfate ions (3 × −2 = −6),
resulting in the formula 𝐹𝑒2 (𝑆𝑂4 )3 .
QUESTION 4
What volume of 0.250 M 𝑁𝑎𝑂𝐻 solution is required to completely
neutralize 50.0 mL of 0.150 M 𝐻𝐶𝑙?
, • A. 20.0 mL
• B. 30.0 mL
• C. 50.0 mL
• D. 83.3 mL
Correct Answer: B. 30.0 mL
Detailed Rationale: The neutralization reaction is 𝐻𝐶𝑙 (𝑎𝑞 ) +
𝑁𝑎𝑂𝐻 (𝑎𝑞 ) → 𝑁𝑎𝐶𝑙 (𝑎𝑞 ) + 𝐻2 𝑂(𝑙), which has a 1: 1 stoichiometric
ratio. Calculate moles of 𝐻𝐶𝑙: 𝑀1 𝑉1 = 0.150 mol/L × 0.0500 L =
0.00750 moles. This requires 0.00750 moles of 𝑁𝑎𝑂𝐻. The volume of
𝑁𝑎𝑂𝐻 is calculated as Moles/Molarity = 0.00750 mol/0.250 mol/L =
0.0300 L, or 30.0 mL.
QUESTION 5
A sample of gas occupies 3.20 L at 25∘ C and 1.20 atm. How many moles
of gas are present in the sample? (𝑅 = 0.08206 L$\cdot𝑎𝑡𝑚/
𝑚𝑜𝑙\cdot$K)
• A. 0.128 mol
• B. 0.157 mol
• C. 1.28 mol
• D. 3.10 mol
Correct Answer: B. 0.157 mol
Detailed Rationale: Use the ideal gas law equation 𝑃𝑉 = 𝑛𝑅𝑇. Convert
temperature to Kelvin: 𝑇 = 25 + 273.15 = 298.15 K. Rearrange for
𝑃𝑉 1.20 atm×3.20 L
moles (𝑛): 𝑛 = = = 0.157 mol.
𝑅𝑇 0.08206 L⋅atm/mol⋅K×298.15 K
, QUESTION 6
When 2.50 g of a solid dissolves in 100.0 g of water in a calorimeter,
the temperature of the water drops from 22.5∘ C to 18.1∘ C. What can
be concluded about this dissolution process?
• A. It is exothermic, and Δ𝐻 is positive.
• B. It is endothermic, and Δ𝐻 is positive.
• C. It is exothermic, and Δ𝐻 is negative.
• D. It is endothermic, and Δ𝐻 is negative.
Correct Answer: B. It is endothermic, and Δ𝐻 is positive.
Detailed Rationale: A decrease in the temperature of the water
indicates that the dissolving process absorbed heat from its
surroundings, making it endothermic. For an endothermic process, the
system absorbs heat, meaning the change in enthalpy (Δ𝐻) is positive.
QUESTION 7
What is the ground-state electron configuration of a neutral chromium
atom (𝐶𝑟, atomic number 24)?
• A. [𝐴𝑟]4𝑠 2 3𝑑 4
• B. [𝐴𝑟]4𝑠1 3𝑑 5
• C. [𝐴𝑟]4𝑠 2 3𝑑 9
• D. [𝐴𝑟]4𝑠 0 3𝑑 6
Correct Answer: B. [𝐴𝑟]4𝑠1 3𝑑5
Detailed Rationale: Chromium is an exception to the standard filling
order due to the extra stability associated with half-filled 𝑑-subshells.
