BMST Chemical Reactions and Stoichiometry 3 — Questions and Answers
Question 1: What is a limiting reagent in a chemical reaction?
- The reactant present in the largest amount
- The reactant that is completely consumed first and stops the reaction (Correct answer)
- The product that forms in the smallest quantity
- The substance that speeds up the reaction
Correct answer: The reactant that is completely consumed first and stops the reaction
The limiting reagent is consumed first and determines the maximum amount of product that can form.
Question 2: Balance the equation: _Fe + _O₂ → _Fe₂O₃. What are the correct coefficients?
- 1, 1, 1
- 4, 3, 2 (Correct answer)
- 2, 3, 1
- 3, 2, 2
Correct answer: 4, 3, 2
4Fe + 3O₂ → 2Fe₂O₃ balances with 4 iron and 6 oxygen atoms on each side.
Question 3: In a double replacement reaction AB + CD → AD + CB, what happens?
- One element replaces another in a compound
- Two ionic compounds exchange partners (Correct answer)
- A compound breaks into elements
- Two elements combine to form a compound
Correct answer: Two ionic compounds exchange partners
Double replacement reactions involve two ionic compounds swapping their cations or anions with each other.
Question 4: The percent yield of a reaction is calculated as:
- (theoretical yield / actual yield) × 100
- (actual yield / theoretical yield) × 100 (Correct answer)
- (actual yield − theoretical yield) × 100
- actual yield × theoretical yield × 100
Correct answer: (actual yield / theoretical yield) × 100
Percent yield = (actual yield ÷ theoretical yield) × 100, showing efficiency of the reaction.
Question 5: How many molecules are in 1 mole of any substance?
- 1.00 × 10²³
- 6.02 × 10²³ (Correct answer)
- 6.02 × 10²⁴
- 3.01 × 10²³
Correct answer: 6.02 × 10²³
One mole contains 6.02 × 10²³ particles, known as Avogadro's number.
Question 6: Which of the following reactions is a combustion reaction?
- NaOH + HCl → NaCl + H₂O
- C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (Correct answer)
- 2KClO₃ → 2KCl + 3O₂
- Zn + 2HCl → ZnCl₂ + H₂
Correct answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Combustion involves a hydrocarbon reacting with oxygen to produce CO₂ and H₂O; propane burning is a clear example.
Question 7: If you have 0.5 moles of CaCl₂ (molar mass = 111 g/mol), what is its mass?
- 22.2 g
- 55.5 g (Correct answer)
- 111 g
- 222 g
Correct answer: 55.5 g
Mass = 0.5 mol × 111 g/mol = 55.5 g.
BMST Chemical Reactions and Stoichiometry 3
What is a limiting reagent in a chemical reaction?