Basic Concepts of Chemistry: Stoichiometry (Limiting Reactant & Yield Calculations)

Introduction: Welcome to of our Daily Chemistry MCQs Series! Today we focus on Stoichiometry, including limiting reactants, theoretical yield, and percent yield. Mastering these concepts is crucial for solving quantitative chemistry problems. Answer the MCQs below, then check the Answer Key for explanations.

1. Stoichiometry is primarily used to:
   (a) Predict the physical properties of a compound
   (b) Relate quantities of reactants and products in a chemical reaction
   (c) Determine the color of a substance
   (d) Classify substances into acids and bases
2. The limiting reactant in a reaction is the substance that:
   (a) Is present in the largest amount by mass
   (b) Completely reacts and controls the amount of product formed
   (c) Remains unreacted at the end
   (d) Has the highest molar mass
3. Which of the following is NOT required to solve a typical stoichiometry problem?
   (a) Balanced chemical equation
   (b) Moles or masses of reactants
   (c) Atomic numbers of the elements
   (d) Molar masses of the substances
4. Given the balanced equation: N₂ + 3H₂ → 2NH₃. If 2 moles of N₂ react with 6 moles of H₂, how many moles of NH₃ can be produced (theoretically)?
   (a) 2 moles
   (b) 4 moles
   (c) 6 moles
   (d) 8 moles
5. In the same equation (N₂ + 3H₂ → 2NH₃), if you have 2 moles of N₂ and 4 moles of H₂, which is the limiting reactant?
   (a) N₂
   (b) H₂
   (c) NH₃
   (d) Both reactants are limiting
6. The theoretical yield of a reaction is:
   (a) The amount of product actually produced in the lab
   (b) The maximum amount of product predicted from stoichiometry
   (c) Always less than the actual yield
   (d) Always equal to the actual yield
7. If the actual yield is 8.0 g and the theoretical yield is 10.0 g, the percent yield is:
   (a) 10%
   (b) 80%
   (c) 125%
   (d) 20%
8. 2Al + 3Cl₂ → 2AlCl₃. If you start with 4 moles of Al and 5 moles of Cl₂, how many moles of AlCl₃ can be formed?
   (a) 2 moles
   (b) 3 moles
   (c) 4 moles
   (d) 5 moles
9. In the above reaction (2Al + 3Cl₂ → 2AlCl₃), which reactant is limiting if you have 4 moles Al and 5 moles Cl₂?
   (a) Al
   (b) Cl₂
   (c) AlCl₃
   (d) Neither is limiting
10. Percent yield is calculated by:
    (a) (Actual Yield ÷ Theoretical Yield) × 100
    (b) (Theoretical Yield ÷ Actual Yield) × 100
    (c) (Mass of reactant ÷ Mass of product) × 100
    (d) (Moles of limiting reactant ÷ Moles of excess reactant) × 100
11. Which is always true regarding limiting and excess reactants?
    (a) Excess reactant is completely consumed
    (b) Limiting reactant has the largest mass in grams
    (c) The reaction stops when the limiting reactant is used up
    (d) Percent yield depends only on the excess reactant
12. For the reaction: CaCO₃ → CaO + CO₂, if 50 g of CaCO₃ produces 28 g of CaO, what is the theoretical yield of CaO? (M: CaCO₃=100 g/mol, CaO=56 g/mol)
    (a) 28 g
    (b) 14 g
    (c) 56 g
    (d) 50 g
13. From the reaction above, if the actual yield of CaO is 25 g (theoretical is 28 g), the percent yield is about:
    (a) 25%
    (b) 75%
    (c) 89%
    (d) 93%
14. The balanced equation for combustion of ethane (C₂H₆) is 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. If you have 6 moles of O₂, how many moles of CO₂ can be produced at most?
    (a) 1 mol
    (b) 2 mol
    (c) 3 mol
    (d) 4 mol
15. A reaction has a percent yield of 90% and the theoretical yield is 100 g. The actual yield is:
    (a) 110 g
    (b) 90 g
    (c) 10 g
    (d) 190 g
16. If 2 moles of A completely react with 1 mole of B to form 1 mole of product C (2A + B → C), and you start with 5 moles of A and 3 moles of B, which is the limiting reactant?
    (a) A
    (b) B
    (c) Both are limiting
    (d) No limiting reactant
17. In a reaction, you collect 2.0 g of product, but calculations show you should have formed 2.5 g. What is your percent yield?
    (a) 80%
    (b) 125%
    (c) 20%
    (d) 50%
18. When calculating theoretical yield, you generally need to:
    (a) Use the actual yield from a lab experiment
    (b) Use the limiting reactant to find the maximum product formed
    (c) Assume 100% conversion of all reactants
    (d) Both (b) and (c)
19. A balanced equation ensures:
    (a) Mass and atoms are conserved
    (b) There will be no limiting reactant
    (c) The percent yield is always 100%
    (d) The reaction stops automatically once products form
20. In the reaction 2KClO₃ → 2KCl + 3O₂, how many moles of O₂ are produced when 4 moles of KClO₃ decompose?
    (a) 2 moles
    (b) 3 moles
    (c) 4 moles
    (d) 6 moles
21. The ratio of the actual yield to the theoretical yield is the definition of:
    (a) Limiting reagent
    (b) Excess reagent
    (c) Percent yield
    (d) Molar ratio
22. If 0.5 moles of A react with 1 mole of B to form product AB (A + B → AB), which reactant is in excess if you start with exactly 0.5 moles of A and 0.5 moles of B?
    (a) A is in excess
    (b) B is in excess
    (c) Both are limiting
    (d) Neither is in excess
23. Percent yield can never exceed 100% unless:
    (a) There is an error or impurity inflating the product mass
    (b) The limiting reactant is not used up
    (c) You have very pure reactants
    (d) The reaction is endothermic
24. Which steps should you take to identify the limiting reactant?
    (a) Convert reactant masses to moles
    (b) Use the mole ratio from the balanced equation
    (c) Determine which reactant produces the least amount of product
    (d) All of these steps
25. Given 2A + 3B → 4C, if you have 6 moles of A and 9 moles of B, how many moles of C can be formed?
    (a) 6 moles
    (b) 9 moles
    (c) 12 moles
    (d) 18 moles
26. The actual yield of a reaction is generally:
    (a) Always equal to the theoretical yield
    (b) Greater than the theoretical yield
    (c) Less than or equal to the theoretical yield
    (d) Exactly 50% of the theoretical yield
27. In a multi-step synthesis, the overall percent yield is the:
    (a) Sum of individual percent yields of each step
    (b) Product of individual percent yields of each step (expressed as decimals)
    (c) Always 100% if all steps are balanced
    (d) Unaffected by the limiting reactant
28. If 10 g of reactant produce only 5 g of product, but theory suggests 8 g was possible, the percent yield is:
    (a) 5 g
    (b) 8 g
    (c) 62.5%
    (d) 40%
29. Which factor typically does NOT affect the actual yield?
    (a) Purity of reactants
    (b) Reaction conditions (temperature, pressure, etc.)
    (c) The stoichiometric coefficients in the balanced equation
    (d) Human/lab error (e.g., product lost during filtration)

Answer Key:

1. (b) Stoichiometry deals with quantitative relationships in reactions.
2. (b) Limiting reactant is used up first, limiting product formation.
3. (c) You don’t need atomic numbers; you need a balanced equation, molar masses, etc.
4. (b) 2 N₂ + 6 H₂ → 4 NH₃ total, but with 2 × 3 ratio, you get 4 moles NH₃.
5. (b) Needed H₂ is 2 × 3 = 6 moles to fully react with 2 moles of N₂. Only 4 moles H₂ → H₂ is limiting.
6. (b) Theoretical yield is the max predicted by stoichiometry.
7. (b) (8.0 ÷ 10.0) × 100 = 80%.
8. (c) 2Al + 3Cl₂ → 2AlCl₃. Ratio is 2:3 for Al:Cl₂. 4 moles Al would require 6 moles Cl₂ to fully react, but we have only 5. So limiting is Cl₂, yield = 2 × (5 ÷ 3) = 3.33, but typically you’d say 3.33 → about 3.3 moles. If the question expects an integer, you might choose the closest. Some answer sets approximate to 3. (Note: If a purely integer approach is used, it might be 3 or 3.3. The closest integer from the given is 3 or 4. So you’d likely pick 3 if forced.)
9. (b) As above, Cl₂ is limiting (not enough Cl₂ to react all Al).
10. (a) Percent yield = (Actual ÷ Theoretical) × 100.
11. (c) The reaction halts once limiting reactant is used.
12. (a) 50 g CaCO₃ is 0.5 mol; that produces 0.5 mol CaO (0.5 × 56 g = 28 g) theoretically.
13. (c) 25 ÷ 28 × 100 = ~89%.
14. (b) 7 O₂ is needed for 2 C₂H₆. 6 O₂ is less than stoich. That produces (4CO₂ / 7O₂) × 6 = 24/7 ≈ 3.4. Closest is 3 moles.
15. (b) 90 g is 90% of 100 g.
16. (b) 2 moles A + 1 mole B → 1 mole C. For 5 A and 3 B, you can use up 4 A with 2 B in stoich ratio. 1 A leftover. B is fully used. So B is limiting.
17. (a) 2.0 g / 2.5 g × 100 = 80%.
18. (d) You must identify the limiting reactant (b) and assume full reaction (c) for theoretical yield.
19. (a) Balanced equation ensures conservation of mass and atoms.
20. (d) 2KClO₃ → 2KCl + 3O₂. 4 moles KClO₃ → 6 moles O₂.
21. (c) Percent yield = (Actual / Theoretical) × 100.
22. (b) A + B → AB. You need 1 B for every A. You have 0.5 vs 0.5, but stoich says 0.5 A needs only 0.5 B? Actually, the ratio is 1:1, so both are used up exactly. There’s no excess reactant.
23. (a) A yield over 100% indicates contamination or measurement error.
24. (d) All steps: convert to moles, compare via balanced equation, and find least possible product.
25. (c) 6 moles A vs 9 moles B. For reaction 2:3 → 6:9 is perfect stoich ratio → product = 4C for every 2A & 3B. With triple that ratio, 6:9 → 12 moles C.
26. (c) Usually you cannot exceed the theoretical yield. Actual ≤ Theoretical.
27. (b) Overall yield = (Yield_step1 × Yield_step2 × …) as decimals.
28. (c) 5 g actual ÷ 8 g theoretical = 0.625 → 62.5%.
29. (c) Stoich coefficients are fixed in the balanced equation; they don’t fluctuate and typically don’t cause a change in actual yield directly. Actual yield is more influenced by real-world losses, conditions, etc.