CHEM 121 General Chemistry - Stoichiometry and Moles Exam
2026/2027 UPDATE
1. How many atoms are in one mole of any substance?
A. 6.022 x 10^23
B. 6.022 x 10^22
C. 1.204 x 10^23
D. 3.011 x 10^23
Answer: A
Rationale: One mole of any substance contains Avogadro’s number of particles, which is
approximately 6.022 x 10^23.
2. What is the molar mass of Water (H2O)? (H=1.01, O=16.00)
A. 17.01 g/mol
B. 18.02 g/mol
C. 16.00 g/mol
D. 33.01 g/mol
Answer: B
Rationale: Molar mass of H2O = (2 * 1.01) + 16.00 = 18.02 g/mol.
3. In the reaction 2H2 + O2 -> 2H2O, what is the mole ratio of H2 to H2O?
A. 1:1
B. 2:1
C. 1:2
D. 2:3
Answer: A
,Rationale: The coefficients are 2 for H2 and 2 for H2O, which simplifies to a 1:1 ratio.
4. Which reactant is entirely consumed first in a chemical reaction?
A. Excess reagent
B. Limiting reagent
C. Catalyst
D. Product
Answer: B
Rationale: The limiting reagent is the reactant that is completely used up first, limiting the
amount of product formed.
5. What is the mass of 2.50 moles of Oxygen gas (O2)? (O=16.00)
A. 80.0 g
B. 16.0 g
C. 40.0 g
D. 64.0 g
Answer: A
Rationale: Mass = moles * molar mass. O2 molar mass is 32.00 g/mol. 2.50 * 32.00 = 80.0 g.
6. When balancing the equation C3H8 + O2 -> CO2 + H2O, what is the coefficient
for O2?
A. 3
B. 4
C. 2
D. 5
Answer: D
Rationale: The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O.
, 7. Which formula represents the simplest whole-number ratio of atoms in a
compound?
A. Empirical formula
B. Structural formula
C. Molecular formula
D. Chemical formula
Answer: A
Rationale: The empirical formula gives the simplest ratio of elements in a compound.
8. If the theoretical yield is 50.0g and the actual yield is 45.0g, what is the
percent yield?
A. 5.0%
B. 111%
C. 45.0%
D. 90.0%
Answer: D
Rationale: Percent yield = (Actual / Theoretical) * 100. (45/50) * 100 = 90%.
9. What is the molar mass of Sodium Chloride (NaCl)? (Na=22.99, Cl=35.45)
A. 70.90 g/mol
B. 44.01 g/mol
C. 50.00 g/mol
D. 58.44 g/mol
Answer: D
Rationale: Molar mass of NaCl = 22.99 + 35.45 = 58.44 g/mol.
2026/2027 UPDATE
1. How many atoms are in one mole of any substance?
A. 6.022 x 10^23
B. 6.022 x 10^22
C. 1.204 x 10^23
D. 3.011 x 10^23
Answer: A
Rationale: One mole of any substance contains Avogadro’s number of particles, which is
approximately 6.022 x 10^23.
2. What is the molar mass of Water (H2O)? (H=1.01, O=16.00)
A. 17.01 g/mol
B. 18.02 g/mol
C. 16.00 g/mol
D. 33.01 g/mol
Answer: B
Rationale: Molar mass of H2O = (2 * 1.01) + 16.00 = 18.02 g/mol.
3. In the reaction 2H2 + O2 -> 2H2O, what is the mole ratio of H2 to H2O?
A. 1:1
B. 2:1
C. 1:2
D. 2:3
Answer: A
,Rationale: The coefficients are 2 for H2 and 2 for H2O, which simplifies to a 1:1 ratio.
4. Which reactant is entirely consumed first in a chemical reaction?
A. Excess reagent
B. Limiting reagent
C. Catalyst
D. Product
Answer: B
Rationale: The limiting reagent is the reactant that is completely used up first, limiting the
amount of product formed.
5. What is the mass of 2.50 moles of Oxygen gas (O2)? (O=16.00)
A. 80.0 g
B. 16.0 g
C. 40.0 g
D. 64.0 g
Answer: A
Rationale: Mass = moles * molar mass. O2 molar mass is 32.00 g/mol. 2.50 * 32.00 = 80.0 g.
6. When balancing the equation C3H8 + O2 -> CO2 + H2O, what is the coefficient
for O2?
A. 3
B. 4
C. 2
D. 5
Answer: D
Rationale: The balanced equation is C3H8 + 5O2 -> 3CO2 + 4H2O.
, 7. Which formula represents the simplest whole-number ratio of atoms in a
compound?
A. Empirical formula
B. Structural formula
C. Molecular formula
D. Chemical formula
Answer: A
Rationale: The empirical formula gives the simplest ratio of elements in a compound.
8. If the theoretical yield is 50.0g and the actual yield is 45.0g, what is the
percent yield?
A. 5.0%
B. 111%
C. 45.0%
D. 90.0%
Answer: D
Rationale: Percent yield = (Actual / Theoretical) * 100. (45/50) * 100 = 90%.
9. What is the molar mass of Sodium Chloride (NaCl)? (Na=22.99, Cl=35.45)
A. 70.90 g/mol
B. 44.01 g/mol
C. 50.00 g/mol
D. 58.44 g/mol
Answer: D
Rationale: Molar mass of NaCl = 22.99 + 35.45 = 58.44 g/mol.
