Chemistry Test Stoichiometry 2 — Questions and Answers
Question 1: How many grams of water are produced when 4 grams of hydrogen gas (H₂) reacts completely with excess oxygen? (H=1, O=16)
- 18 g
- 36 g (Correct answer)
- 72 g
- 9 g
Correct answer: 36 g
4 g H₂ = 2 mol H₂, and 2H₂ + O₂ → 2H₂O gives 2 mol H₂O = 36 g.
Question 2: What is the percent yield if a reaction theoretically produces 50 g of product but only 38 g is actually obtained?
- 76% (Correct answer)
- 68%
- 82%
- 56%
Correct answer: 76%
Percent yield = (actual/theoretical) × 100 = (38/50) × 100 = 76%.
Question 3: In the reaction N₂ + 3H₂ → 2NH₃, if 6 mol of H₂ is used, how many moles of NH₃ are produced?
- 2 mol
- 4 mol (Correct answer)
- 6 mol
- 3 mol
Correct answer: 4 mol
6 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 4 mol NH₃.
Question 4: A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula? (C=12, H=1, O=16)
- CH₂O (Correct answer)
- C₂H₄O₂
- CHO
- C₂H₂O
Correct answer: CH₂O
Dividing by atomic masses: C=40/12≈3.33, H=6.7/1=6.7, O=53.3/16≈3.33; ratio 1:2:1 gives CH₂O.
Question 5: Which quantity is conserved during a chemical reaction according to the law of conservation of mass?
- Number of molecules
- Total mass of reactants and products (Correct answer)
- Number of moles
- Volume of gases
Correct answer: Total mass of reactants and products
The law of conservation of mass states that total mass of reactants equals total mass of products.
Question 6: If 10 g of CaCO₃ decomposes completely (CaCO₃ → CaO + CO₂), how many grams of CO₂ are released? (Ca=40, C=12, O=16)
- 4.4 g (Correct answer)
- 5.6 g
- 8.8 g
- 2.2 g
Correct answer: 4.4 g
10 g CaCO₃ = 0.1 mol; produces 0.1 mol CO₂ = 0.1 × 44 = 4.4 g.
Question 7: What does the term 'stoichiometric coefficients' refer to in a balanced chemical equation?
- The atomic masses of each element
- The numbers placed before formulas indicating mole ratios (Correct answer)
- The oxidation states of elements
- The energy released per mole
Correct answer: The numbers placed before formulas indicating mole ratios
Stoichiometric coefficients are the numbers in front of chemical formulas that show mole ratios of reactants and products.
Chemistry Test Stoichiometry 2
How many grams of water are produced when 4 grams of hydrogen gas (H₂) reacts completely with excess oxygen? (H=1, O=16)