CHEM 101 Fall 2025 Exam 3 Version A | Complete
Questions and Correct Answers | Verified Answer
Key.
CHEM 101 Fall 2025 Exam 3 Version A
COMPLETE PRACTICE EXAM: ALL 100 QUESTIONS WITH VERIFIED ANSWERS
PART I:
1. Please choose the letter "A" as your answer for this question.
A) A
B) B
C) C
D) D
E) E
Rationale: This is a standard instruction question included to ensure you are
reading the directions carefully and filling in your answer sheet correctly. The
answer is A .
2. Choose the correct chemical formula for the compound formed when calcium
reacts with sulfur.
A) CaS
B) CaS₂
C) Ca₂S₃
D) Ca₃S₂
E) Ca₂S
,Rationale: Calcium (Ca²⁺) and sulfur (S²⁻) combine in a 1:1 ratio to form CaS. The
charges balance: Ca²⁺ + S²⁻ → CaS .
3. Determine the total number of valence electrons that must be considered to
write the correct Lewis structures for NH₄⁺ and HSO₄⁻, respectively.
A) 9; 31
B) 8; 32
C) 10; 29
D) 8; 31
E) 18; 10
Rationale: NH₄⁺: N (5) + 4H (4×1) - 1 (positive charge) = 8 valence electrons. HSO₄⁻:
H (1) + S (6) + 4O (4×6) + 1 (negative charge) = 31 valence electrons .
4. How many millimoles of calcium chloride, CaCl₂, contain 4.78 × 10²² formula
units of this substance?
A) 12.6 mmol
B) 13.0 mmol
C) 20.7 mmol
D) 79.4 mmol
E) 57.0 mmol
Rationale: Use Avogadro's number: (4.78 × 10²²) / (6.022 × 10²³) = 0.0794 mol =
79.4 mmol .
5. Which of the following substances is a non-electrolyte when dissolved in
water?
A) LiOH
B) Na₂HPO₄
C) NH₄Cl
,D) NaCl
E) CH₃OH
Rationale: CH₃OH (methanol) is a covalent compound that does not dissociate into
ions in water. The other options are ionic compounds or strong electrolytes .
6. Which of the following correctly describes the charge on the arsenic atom in
an As-Br bond?
A) A full -1 charge representing the transfer of one electron from bromine to
arsenic
B) A partial negative charge representing the higher effective nuclear charge in the
As atom
C) A partial positive charge representing the electron-deficient side of the As-Br
bond
D) A full +1 charge representing the transfer of one electron from arsenic to
bromine
E) No charge representing a uniform distribution of the bonding electrons
Rationale: Bromine is more electronegative than arsenic, so it pulls electron
density toward itself. The arsenic atom becomes electron-deficient and carries a
partial positive charge (δ⁺) .
7. Estimate the mass percent of potassium in potassium sulfate, K₂SO₄.
A) 22.4%
B) 44.9%
C) 28.9%
D) 57.9%
E) 21.8%
Rationale: Molar mass of K₂SO₄ = 2(39.10) + 32.07 + 4(16.00) = 174.27 g/mol. %K
= (78.20/174.27) × 100 = 44.87% ≈ 44.9% .
, 8. How many unpaired electrons does the Lewis symbol for Se show?
A) 6
B) 2
C) 4
D) 3
E) 1
Rationale: Selenium (Se) is in Group 6A with 6 valence electrons. Its Lewis symbol
shows two paired electrons (in one orbital) and two unpaired electrons (in two
separate orbitals) .
9. The recommended daily dose for iodine is 150 μg/day. How much potassium
iodide, KI (76.45% I), must you consume to meet that requirement?
A) 196 μg
B) 0.511 μg
C) 11.5 μg
D) 127 μg
E) 16.6 μg
Rationale: If KI is 76.45% iodine by mass, then 150 μg = 0.7645 × (mass of KI).
Mass KI = 150/0.7645 = 196 μg .
10. Potassium hydrogen phosphate, K₂HPO₄, is used as food additive, fertilizer,
and in the pharmaceutical industry. Which of the following ionic dissociations
shows correctly what happens when this substance is added to water?
A) K⁺ + HPO₄⁻
B) 2 K⁺ + HPO₄²⁻
C) K₂⁺ + 2 HPO₄⁻
Questions and Correct Answers | Verified Answer
Key.
CHEM 101 Fall 2025 Exam 3 Version A
COMPLETE PRACTICE EXAM: ALL 100 QUESTIONS WITH VERIFIED ANSWERS
PART I:
1. Please choose the letter "A" as your answer for this question.
A) A
B) B
C) C
D) D
E) E
Rationale: This is a standard instruction question included to ensure you are
reading the directions carefully and filling in your answer sheet correctly. The
answer is A .
2. Choose the correct chemical formula for the compound formed when calcium
reacts with sulfur.
A) CaS
B) CaS₂
C) Ca₂S₃
D) Ca₃S₂
E) Ca₂S
,Rationale: Calcium (Ca²⁺) and sulfur (S²⁻) combine in a 1:1 ratio to form CaS. The
charges balance: Ca²⁺ + S²⁻ → CaS .
3. Determine the total number of valence electrons that must be considered to
write the correct Lewis structures for NH₄⁺ and HSO₄⁻, respectively.
A) 9; 31
B) 8; 32
C) 10; 29
D) 8; 31
E) 18; 10
Rationale: NH₄⁺: N (5) + 4H (4×1) - 1 (positive charge) = 8 valence electrons. HSO₄⁻:
H (1) + S (6) + 4O (4×6) + 1 (negative charge) = 31 valence electrons .
4. How many millimoles of calcium chloride, CaCl₂, contain 4.78 × 10²² formula
units of this substance?
A) 12.6 mmol
B) 13.0 mmol
C) 20.7 mmol
D) 79.4 mmol
E) 57.0 mmol
Rationale: Use Avogadro's number: (4.78 × 10²²) / (6.022 × 10²³) = 0.0794 mol =
79.4 mmol .
5. Which of the following substances is a non-electrolyte when dissolved in
water?
A) LiOH
B) Na₂HPO₄
C) NH₄Cl
,D) NaCl
E) CH₃OH
Rationale: CH₃OH (methanol) is a covalent compound that does not dissociate into
ions in water. The other options are ionic compounds or strong electrolytes .
6. Which of the following correctly describes the charge on the arsenic atom in
an As-Br bond?
A) A full -1 charge representing the transfer of one electron from bromine to
arsenic
B) A partial negative charge representing the higher effective nuclear charge in the
As atom
C) A partial positive charge representing the electron-deficient side of the As-Br
bond
D) A full +1 charge representing the transfer of one electron from arsenic to
bromine
E) No charge representing a uniform distribution of the bonding electrons
Rationale: Bromine is more electronegative than arsenic, so it pulls electron
density toward itself. The arsenic atom becomes electron-deficient and carries a
partial positive charge (δ⁺) .
7. Estimate the mass percent of potassium in potassium sulfate, K₂SO₄.
A) 22.4%
B) 44.9%
C) 28.9%
D) 57.9%
E) 21.8%
Rationale: Molar mass of K₂SO₄ = 2(39.10) + 32.07 + 4(16.00) = 174.27 g/mol. %K
= (78.20/174.27) × 100 = 44.87% ≈ 44.9% .
, 8. How many unpaired electrons does the Lewis symbol for Se show?
A) 6
B) 2
C) 4
D) 3
E) 1
Rationale: Selenium (Se) is in Group 6A with 6 valence electrons. Its Lewis symbol
shows two paired electrons (in one orbital) and two unpaired electrons (in two
separate orbitals) .
9. The recommended daily dose for iodine is 150 μg/day. How much potassium
iodide, KI (76.45% I), must you consume to meet that requirement?
A) 196 μg
B) 0.511 μg
C) 11.5 μg
D) 127 μg
E) 16.6 μg
Rationale: If KI is 76.45% iodine by mass, then 150 μg = 0.7645 × (mass of KI).
Mass KI = 150/0.7645 = 196 μg .
10. Potassium hydrogen phosphate, K₂HPO₄, is used as food additive, fertilizer,
and in the pharmaceutical industry. Which of the following ionic dissociations
shows correctly what happens when this substance is added to water?
A) K⁺ + HPO₄⁻
B) 2 K⁺ + HPO₄²⁻
C) K₂⁺ + 2 HPO₄⁻
