CHEM 120 Exam Prep | 350+ Practice QUESTIONs
with Correct Answers & Detailed Explanations
QUESTION 1
Which of the following best describes an isotope of an element?
• A. Atoms of the same element that have different numbers of
protons.
• B. Atoms of the same element that have the same number of
neutrons but different mass numbers.
• C. Atoms of the same element that have the same number of
protons but different numbers of neutrons.
• D. Atoms with the same mass number but different atomic
numbers.
Correct Answer: C. Atoms of the same element that have the same
number of protons but different numbers of neutrons.
Detailed Rationale: Isotopes are defined as atoms belonging to the
same element (sharing an identical atomic number or number of
protons) that differ in their mass numbers because they contain a
different number of neutrons.
QUESTION 2
How does electronegativity generally change across the periodic table?
• A. It increases from left to right across a period and decreases
from top to bottom down a group.
, • B. It decreases from left to right across a period and increases
from top to bottom down a group.
• C. It remains constant across a period and increases down a group.
• D. It increases from left to right across a period and increases from
top to bottom down a group.
Correct Answer: A. It increases from left to right across a period and
decreases from top to bottom down a group.
Detailed Rationale: Electronegativity increases across a period from left
to right due to an increasing effective nuclear charge that attracts
bonding electrons more strongly. It decreases down a group as atomic
radius increases and valence electrons become further from the
nucleus, shielded by inner electron shells.
QUESTION 3
What is the mass in grams of 2.50 moles of water (H₂O)?
• A. 18.02 g
• B. 36.04 g
• C. 45.05 g
• D. 4.51 g
Correct Answer: C. 45.05 g
Detailed Rationale: The molar mass of water (H₂O) is calculated as
2(1.008) + 16.00 = 18.02 g/mol. Multiplying the number of moles by
the molar mass (2.50 mol × 18.02 g/mol) yields 45.05 g.
QUESTION 4
,A sample of an ideal gas occupies 4.00 L at 300 K and 1.00 atm. What
will be its volume if the temperature is doubled to 600 K while the
pressure remains constant?
• A. 2.00 L
• B. 4.00 L
• C. 8.00 L
• D. 16.00 L
Correct Answer: C. 8.00 L
Detailed Rationale: According to Charles's Law, volume is directly
proportional to absolute temperature at a constant pressure (𝑉1 /𝑇1 =
𝑉2 /𝑇2 ). Doubling the absolute temperature from 300 K to 600 K doubles
the volume from 4.00 L to 8.00 L.
QUESTION 5
What is the molecular geometry of a methane molecule (CH₄) according
to VSEPR theory?
• A. Linear
• B. Trigonal planar
• C. Tetrahedral
• D. Bent
Correct Answer: C. Tetrahedral
Detailed Rationale: Methane contains a central carbon atom bonded to
four hydrogen atoms with zero lone pairs. To minimize electrostatic
repulsion between these four electron pairs, the molecule adopts a
, three-dimensional tetrahedral geometry with bond angles of
approximately 109.5°.
QUESTION 6
When a system releases heat to its surroundings, the process is
classified as:
• A. Endothermic, with a positive enthalpy change.
• B. Exothermic, with a negative enthalpy change.
• C. Endothermic, with a negative enthalpy change.
• D. Exothermic, with a positive enthalpy change.
Correct Answer: B. Exothermic, with a negative enthalpy change.
Detailed Rationale: In an exothermic process, thermal energy flows out
of the system into the surroundings. Because the system loses energy,
the change in enthalpy (Δ𝐻) is negative.
QUESTION 7
What is the molarity of a solution containing 0.500 moles of solute
dissolved in 250 mL of solution?
• A. 0.125 M
• B. 2.00 M
• C. 0.500 M
• D. 4.00 M
Correct Answer: B. 2.00 M
with Correct Answers & Detailed Explanations
QUESTION 1
Which of the following best describes an isotope of an element?
• A. Atoms of the same element that have different numbers of
protons.
• B. Atoms of the same element that have the same number of
neutrons but different mass numbers.
• C. Atoms of the same element that have the same number of
protons but different numbers of neutrons.
• D. Atoms with the same mass number but different atomic
numbers.
Correct Answer: C. Atoms of the same element that have the same
number of protons but different numbers of neutrons.
Detailed Rationale: Isotopes are defined as atoms belonging to the
same element (sharing an identical atomic number or number of
protons) that differ in their mass numbers because they contain a
different number of neutrons.
QUESTION 2
How does electronegativity generally change across the periodic table?
• A. It increases from left to right across a period and decreases
from top to bottom down a group.
, • B. It decreases from left to right across a period and increases
from top to bottom down a group.
• C. It remains constant across a period and increases down a group.
• D. It increases from left to right across a period and increases from
top to bottom down a group.
Correct Answer: A. It increases from left to right across a period and
decreases from top to bottom down a group.
Detailed Rationale: Electronegativity increases across a period from left
to right due to an increasing effective nuclear charge that attracts
bonding electrons more strongly. It decreases down a group as atomic
radius increases and valence electrons become further from the
nucleus, shielded by inner electron shells.
QUESTION 3
What is the mass in grams of 2.50 moles of water (H₂O)?
• A. 18.02 g
• B. 36.04 g
• C. 45.05 g
• D. 4.51 g
Correct Answer: C. 45.05 g
Detailed Rationale: The molar mass of water (H₂O) is calculated as
2(1.008) + 16.00 = 18.02 g/mol. Multiplying the number of moles by
the molar mass (2.50 mol × 18.02 g/mol) yields 45.05 g.
QUESTION 4
,A sample of an ideal gas occupies 4.00 L at 300 K and 1.00 atm. What
will be its volume if the temperature is doubled to 600 K while the
pressure remains constant?
• A. 2.00 L
• B. 4.00 L
• C. 8.00 L
• D. 16.00 L
Correct Answer: C. 8.00 L
Detailed Rationale: According to Charles's Law, volume is directly
proportional to absolute temperature at a constant pressure (𝑉1 /𝑇1 =
𝑉2 /𝑇2 ). Doubling the absolute temperature from 300 K to 600 K doubles
the volume from 4.00 L to 8.00 L.
QUESTION 5
What is the molecular geometry of a methane molecule (CH₄) according
to VSEPR theory?
• A. Linear
• B. Trigonal planar
• C. Tetrahedral
• D. Bent
Correct Answer: C. Tetrahedral
Detailed Rationale: Methane contains a central carbon atom bonded to
four hydrogen atoms with zero lone pairs. To minimize electrostatic
repulsion between these four electron pairs, the molecule adopts a
, three-dimensional tetrahedral geometry with bond angles of
approximately 109.5°.
QUESTION 6
When a system releases heat to its surroundings, the process is
classified as:
• A. Endothermic, with a positive enthalpy change.
• B. Exothermic, with a negative enthalpy change.
• C. Endothermic, with a negative enthalpy change.
• D. Exothermic, with a positive enthalpy change.
Correct Answer: B. Exothermic, with a negative enthalpy change.
Detailed Rationale: In an exothermic process, thermal energy flows out
of the system into the surroundings. Because the system loses energy,
the change in enthalpy (Δ𝐻) is negative.
QUESTION 7
What is the molarity of a solution containing 0.500 moles of solute
dissolved in 250 mL of solution?
• A. 0.125 M
• B. 2.00 M
• C. 0.500 M
• D. 4.00 M
Correct Answer: B. 2.00 M
