CHEM 103 Module 4 Exam: Chemical Reactions and Stoichiometry
Exam 2026/2027 UPDATE
1. What is the value of Avogadro’s number?
A. 6.022 x 10^22
B. 6.022 x 10^23
C. 3.011 x 10^23
D. 1.000 x 10^24
Answer: B
Rationale: Avogadro’s number is defined as the number of atoms in exactly 12 grams of
Carbon-12, which is 6.022 x 10^23.
2. Which of the following describes a synthesis reaction?
A. Two or more substances combine to form a single product.
B. An element reacts with a compound to produce a new element.
C. A single reactant breaks down into multiple products.
D. Two compounds exchange ions to form new compounds.
Answer: A
Rationale: A synthesis reaction (or combination) occurs when two or more reactants
combine to form one single product (A + B -> AB).
,3. Calculate the molar mass of Calcium Carbonate (CaCO3).
A. 68.09 g/mol
B. 84.00 g/mol
C. 100.09 g/mol
D. 120.12 g/mol
Answer: C
Rationale: Ca (40.08) + C (12.01) + 3*O (16.00) = 100.09 g/mol.
4. In the balanced equation 2H2 + O2 -> 2H2O, what is the mole ratio of H2 to
H2O?
A. 1:2
B. 2:1
C. 1:1
D. 2:3
Answer: C
Rationale: The coefficients for H2 and H2O are both 2, so the ratio is 2:2, which simplifies
to 1:1.
5. What is the mass of 0.500 moles of NaCl?
A. 116.88 g
B. 58.44 g
C. 14.61 g
D. 29.22 g
Answer: D
Rationale: Molar mass of NaCl is 58.44 g/mol. 0.500 moles * 58.44 g/mol = 29.22 g.
, 6. Which type of reaction is 2KClO3 -> 2KCl + 3O2?
A. Decomposition
B. Synthesis
C. Single Replacement
D. Double Replacement
Answer: A
Rationale: A decomposition reaction occurs when a single reactant breaks down into two
or more products.
7. Identify the coefficient of Oxygen when the equation C3H8 + O2 -> CO2 + H2O
is balanced.
A. 3
B. 4
C. 5
D. 10
Answer: C
Rationale: The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O.
8. What is the empirical formula for a compound that is 80% Carbon and 20%
Hydrogen by mass?
A. CH
B. CH3
C. CH2
D. C2H5
Answer: B
Rationale: 80g C / 12 = 6.67 mol; 20g H / 1 = 20 mol. .67 = 3. Thus, CH3.
Exam 2026/2027 UPDATE
1. What is the value of Avogadro’s number?
A. 6.022 x 10^22
B. 6.022 x 10^23
C. 3.011 x 10^23
D. 1.000 x 10^24
Answer: B
Rationale: Avogadro’s number is defined as the number of atoms in exactly 12 grams of
Carbon-12, which is 6.022 x 10^23.
2. Which of the following describes a synthesis reaction?
A. Two or more substances combine to form a single product.
B. An element reacts with a compound to produce a new element.
C. A single reactant breaks down into multiple products.
D. Two compounds exchange ions to form new compounds.
Answer: A
Rationale: A synthesis reaction (or combination) occurs when two or more reactants
combine to form one single product (A + B -> AB).
,3. Calculate the molar mass of Calcium Carbonate (CaCO3).
A. 68.09 g/mol
B. 84.00 g/mol
C. 100.09 g/mol
D. 120.12 g/mol
Answer: C
Rationale: Ca (40.08) + C (12.01) + 3*O (16.00) = 100.09 g/mol.
4. In the balanced equation 2H2 + O2 -> 2H2O, what is the mole ratio of H2 to
H2O?
A. 1:2
B. 2:1
C. 1:1
D. 2:3
Answer: C
Rationale: The coefficients for H2 and H2O are both 2, so the ratio is 2:2, which simplifies
to 1:1.
5. What is the mass of 0.500 moles of NaCl?
A. 116.88 g
B. 58.44 g
C. 14.61 g
D. 29.22 g
Answer: D
Rationale: Molar mass of NaCl is 58.44 g/mol. 0.500 moles * 58.44 g/mol = 29.22 g.
, 6. Which type of reaction is 2KClO3 -> 2KCl + 3O2?
A. Decomposition
B. Synthesis
C. Single Replacement
D. Double Replacement
Answer: A
Rationale: A decomposition reaction occurs when a single reactant breaks down into two
or more products.
7. Identify the coefficient of Oxygen when the equation C3H8 + O2 -> CO2 + H2O
is balanced.
A. 3
B. 4
C. 5
D. 10
Answer: C
Rationale: The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O.
8. What is the empirical formula for a compound that is 80% Carbon and 20%
Hydrogen by mass?
A. CH
B. CH3
C. CH2
D. C2H5
Answer: B
Rationale: 80g C / 12 = 6.67 mol; 20g H / 1 = 20 mol. .67 = 3. Thus, CH3.
