CHEM 103 | CHEM103 Exam 2: General Chemistry
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. When balancing the combustion reaction of propane (C3H8 + O2 -> CO2 + H2O), what is the
coefficient for oxygen (O2)?
A. 3
B. 4
C. 5
D. 10
Correct Answer: C
Expert Explanation: To balance C3H8 + 5O2 -> 3CO2 + 4H2O, we first balance the 3
carbons by putting 3 in front of CO2 and the 8 hydrogens by putting 4 in front of H2O. This
gives a total of 10 oxygen atoms on the product side (6 from CO2 and 4 from H2O). Dividing
10 by 2 gives a coefficient of 5 for O2. This ensures that the Law of Conservation of Mass is
satisfied for all elements involved. This process demonstrates the fundamental
stoichiometric principle of equating atoms across a chemical equation.
2. What is the molar mass of Calcium Carbonate (CaCO3)?
A. 100.09 g/mol
B. 68.09 g/mol
C. 84.00 g/mol
D. 116.12 g/mol
Correct Answer: A
Expert Explanation: The molar mass is calculated by summing the atomic masses: Ca
(40.08) + C (12.01) + 3 * O (16.00) = 100.09 g/mol. Understanding molar mass is crucial for
converting between the mass of a substance and the number of moles.
3. In the reaction 2H2 + O2 -> 2H2O, how many moles of water are produced from 5.0 moles
of oxygen gas, assuming excess hydrogen?
A. 2.5 moles
B. 5.0 moles
C. 10.0 moles
D. 20.0 moles
,Correct Answer: C
Expert Explanation: The stoichiometric ratio between O2 and H2O is 1:2. Therefore, 5.0
moles of O2 will produce 5.0 * 2 = 10.0 moles of H2O. This illustrates mole-to-mole
conversions using balanced equation coefficients.
4. Which of the following defines the ‘Limiting Reactant’ in a chemical reaction?
A. The reactant with the largest mass.
B. The reactant that is completely consumed first.
C. The reactant that remains after the reaction stops.
D. The reactant with the smallest coefficient in the balanced equation.
Correct Answer: B
Expert Explanation: The limiting reactant is the substance that is totally consumed when
the chemical reaction is complete. The amount of product formed is limited by this reactant
because the reaction cannot continue without it.
5. If the theoretical yield of a reaction is 50.0 grams but the actual yield is 40.0 grams, what is
the percent yield?
A. 80%
B. 125%
C. 90%
D. 20%
Correct Answer: A
Expert Explanation: Percent yield is calculated as (Actual Yield / Theoretical Yield) * 100.
In this case, (40..0) * 100 = 80%. This measures the efficiency of a chemical reaction.
6. Balance the following equation: Al + HCl -> AlCl3 + H2. What is the coefficient of HCl?
A. 2
B. 6
C. 3
D. 1
Correct Answer: B
Expert Explanation: The balanced equation is 2Al + 6HCl -> 2AlCl3 + 3H2. This is achieved
by finding a common multiple for Chlorine and Hydrogen atoms on both sides.
, 7. How many grams of CO2 are produced from the combustion of 1.00 mole of methane
(CH4)?
A. 16.04 g
B. 44.01 g
C. 32.00 g
D. 12.01 g
Correct Answer: B
Expert Explanation: From the balanced equation CH4 + 2O2 -> CO2 + 2H2O, 1 mole of CH4
produces 1 mole of CO2. The molar mass of CO2 is 44.01 g/mol, so 1 mole equals 44.01 g.
8. What mass of Magnesium Oxide (MgO) is produced from 24.3 g of Magnesium reacting
with excess Oxygen? (2Mg + O2 -> 2MgO)
A. 24.3 g
B. 80.6 g
C. 40.3 g
D. 48.6 g
Correct Answer: C
Expert Explanation: 24.3 g of Mg is 1 mole. According to the ratio (2:2 or 1:1), 1 mole of
Mg produces 1 mole of MgO. The molar mass of MgO is 24.3 + 16.0 = 40.3 g/mol.
9. Identify the limiting reactant when 10.0 g of H2 reacts with 10.0 g of O2 to form H2O.
A. H2
B. Neither
C. H2O
D. O2
Correct Answer: D
Expert Explanation: Moles of H2 = 10/2 = 5 mol. Moles of O2 = 10/32 = 0.3125 mol. The
reaction requires 2 moles of H2 for every 1 mole of O2. O2 will run out much faster.
10. Which of the following is NOT a reason why actual yield is often less than theoretical
yield?
A. Side reactions occurring.
B. Incomplete reactions.
C. Creation of matter during the reaction.
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. When balancing the combustion reaction of propane (C3H8 + O2 -> CO2 + H2O), what is the
coefficient for oxygen (O2)?
A. 3
B. 4
C. 5
D. 10
Correct Answer: C
Expert Explanation: To balance C3H8 + 5O2 -> 3CO2 + 4H2O, we first balance the 3
carbons by putting 3 in front of CO2 and the 8 hydrogens by putting 4 in front of H2O. This
gives a total of 10 oxygen atoms on the product side (6 from CO2 and 4 from H2O). Dividing
10 by 2 gives a coefficient of 5 for O2. This ensures that the Law of Conservation of Mass is
satisfied for all elements involved. This process demonstrates the fundamental
stoichiometric principle of equating atoms across a chemical equation.
2. What is the molar mass of Calcium Carbonate (CaCO3)?
A. 100.09 g/mol
B. 68.09 g/mol
C. 84.00 g/mol
D. 116.12 g/mol
Correct Answer: A
Expert Explanation: The molar mass is calculated by summing the atomic masses: Ca
(40.08) + C (12.01) + 3 * O (16.00) = 100.09 g/mol. Understanding molar mass is crucial for
converting between the mass of a substance and the number of moles.
3. In the reaction 2H2 + O2 -> 2H2O, how many moles of water are produced from 5.0 moles
of oxygen gas, assuming excess hydrogen?
A. 2.5 moles
B. 5.0 moles
C. 10.0 moles
D. 20.0 moles
,Correct Answer: C
Expert Explanation: The stoichiometric ratio between O2 and H2O is 1:2. Therefore, 5.0
moles of O2 will produce 5.0 * 2 = 10.0 moles of H2O. This illustrates mole-to-mole
conversions using balanced equation coefficients.
4. Which of the following defines the ‘Limiting Reactant’ in a chemical reaction?
A. The reactant with the largest mass.
B. The reactant that is completely consumed first.
C. The reactant that remains after the reaction stops.
D. The reactant with the smallest coefficient in the balanced equation.
Correct Answer: B
Expert Explanation: The limiting reactant is the substance that is totally consumed when
the chemical reaction is complete. The amount of product formed is limited by this reactant
because the reaction cannot continue without it.
5. If the theoretical yield of a reaction is 50.0 grams but the actual yield is 40.0 grams, what is
the percent yield?
A. 80%
B. 125%
C. 90%
D. 20%
Correct Answer: A
Expert Explanation: Percent yield is calculated as (Actual Yield / Theoretical Yield) * 100.
In this case, (40..0) * 100 = 80%. This measures the efficiency of a chemical reaction.
6. Balance the following equation: Al + HCl -> AlCl3 + H2. What is the coefficient of HCl?
A. 2
B. 6
C. 3
D. 1
Correct Answer: B
Expert Explanation: The balanced equation is 2Al + 6HCl -> 2AlCl3 + 3H2. This is achieved
by finding a common multiple for Chlorine and Hydrogen atoms on both sides.
, 7. How many grams of CO2 are produced from the combustion of 1.00 mole of methane
(CH4)?
A. 16.04 g
B. 44.01 g
C. 32.00 g
D. 12.01 g
Correct Answer: B
Expert Explanation: From the balanced equation CH4 + 2O2 -> CO2 + 2H2O, 1 mole of CH4
produces 1 mole of CO2. The molar mass of CO2 is 44.01 g/mol, so 1 mole equals 44.01 g.
8. What mass of Magnesium Oxide (MgO) is produced from 24.3 g of Magnesium reacting
with excess Oxygen? (2Mg + O2 -> 2MgO)
A. 24.3 g
B. 80.6 g
C. 40.3 g
D. 48.6 g
Correct Answer: C
Expert Explanation: 24.3 g of Mg is 1 mole. According to the ratio (2:2 or 1:1), 1 mole of
Mg produces 1 mole of MgO. The molar mass of MgO is 24.3 + 16.0 = 40.3 g/mol.
9. Identify the limiting reactant when 10.0 g of H2 reacts with 10.0 g of O2 to form H2O.
A. H2
B. Neither
C. H2O
D. O2
Correct Answer: D
Expert Explanation: Moles of H2 = 10/2 = 5 mol. Moles of O2 = 10/32 = 0.3125 mol. The
reaction requires 2 moles of H2 for every 1 mole of O2. O2 will run out much faster.
10. Which of the following is NOT a reason why actual yield is often less than theoretical
yield?
A. Side reactions occurring.
B. Incomplete reactions.
C. Creation of matter during the reaction.
