Chemistry Test Stoichiometry 3 — Questions and Answers
Question 1: In the reaction 2Al + 3Cl₂ → 2AlCl₃, if you start with 3 mol Al and 3 mol Cl₂, which is the limiting reagent?
- Al
- Cl₂ (Correct answer)
- AlCl₃
- Neither; both are in excess
Correct answer: Cl₂
3 mol Al needs 4.5 mol Cl₂, but only 3 mol Cl₂ is available, so Cl₂ is limiting.
Question 2: What is the molar mass of glucose (C₆H₁₂O₆)? (C=12, H=1, O=16)
- 120 g/mol
- 180 g/mol (Correct answer)
- 144 g/mol
- 168 g/mol
Correct answer: 180 g/mol
6(12) + 12(1) + 6(16) = 72 + 12 + 96 = 180 g/mol.
Question 3: A reaction produces 25.0 g of NaCl but the theoretical yield was 30.0 g. What is the percent yield?
- 83.3% (Correct answer)
- 75.0%
- 120%
- 66.7%
Correct answer: 83.3%
Percent yield = (25.0/30.0) × 100 = 83.3%.
Question 4: How many molecules are in 0.5 mol of oxygen gas (O₂)? (Avogadro's number = 6.02 × 10²³)
- 3.01 × 10²³ (Correct answer)
- 6.02 × 10²³
- 1.20 × 10²⁴
- 1.51 × 10²³
Correct answer: 3.01 × 10²³
0.5 mol × 6.02 × 10²³ molecules/mol = 3.01 × 10²³ molecules.
Question 5: The empirical formula of a compound is CH₂. If the molar mass is 56 g/mol, what is the molecular formula? (C=12, H=1)
- C₃H₆
- C₄H₈ (Correct answer)
- C₂H₄
- C₅H₁₀
Correct answer: C₄H₈
Empirical formula mass of CH₂ = 14 g/mol; 56/14 = 4, so molecular formula is C₄H₈.
Question 6: In stoichiometry, what does STP stand for and what volume does 1 mole of ideal gas occupy at STP?
- Standard Temperature and Pressure; 22.4 L (Correct answer)
- Standard Test Parameters; 24.5 L
- Standard Temperature and Pressure; 24.5 L
- Standard Test Procedure; 22.4 L
Correct answer: Standard Temperature and Pressure; 22.4 L
STP is Standard Temperature (0°C) and Pressure (1 atm), at which 1 mol of ideal gas occupies 22.4 L.
Question 7: If 5.6 L of CO₂ gas is collected at STP, how many moles of CO₂ were produced? (molar volume at STP = 22.4 L/mol)
- 0.25 mol (Correct answer)
- 0.50 mol
- 0.75 mol
- 1.00 mol
Correct answer: 0.25 mol
5.6 L ÷ 22.4 L/mol = 0.25 mol CO₂.
Chemistry Test Stoichiometry 3
In the reaction 2Al + 3Cl₂ → 2AlCl₃, if you start with 3 mol Al and 3 mol Cl₂, which is the limiting reagent?