RPAL Limiting Reagents 1 — Questions and Answers
Question 1: What is a limiting reagent in a chemical reaction?
- The reactant that is completely consumed and stops the reaction (Correct answer)
- The reactant present in the largest amount
- The product formed in the greatest quantity
- The reagent with the highest molar mass
Correct answer: The reactant that is completely consumed and stops the reaction
The limiting reagent is the reactant that runs out first, thereby determining the maximum amount of product that can be formed.
Question 2: In the reaction 2H₂ + O₂ → 2H₂O, if you have 4 mol H₂ and 1 mol O₂, which is the limiting reagent?
- O₂ (Correct answer)
- H₂
- H₂O
- Neither — they are in perfect ratio
Correct answer: O₂
The reaction requires 2:1 ratio of H₂ to O₂, so 4 mol H₂ needs 2 mol O₂, but only 1 mol O₂ is available, making O₂ the limiting reagent.
Question 3: How do you identify the limiting reagent when given masses of two reactants?
- Convert masses to moles, divide by coefficients, and pick the smallest result (Correct answer)
- Pick the reactant with the smaller mass
- Pick the reactant with the larger molar mass
- Convert masses to moles and pick the largest value
Correct answer: Convert masses to moles, divide by coefficients, and pick the smallest result
Converting each reactant's mass to moles and dividing by its stoichiometric coefficient gives a comparable ratio — the smallest value indicates the limiting reagent.
Question 4: If the limiting reagent is fully consumed, what happens to the excess reagent?
- Some of it remains unreacted (leftover) (Correct answer)
- It is all converted to products
- It decomposes spontaneously
- It becomes the new limiting reagent
Correct answer: Some of it remains unreacted (leftover)
Once the limiting reagent is used up, the reaction stops and the excess reagent remains partially unreacted as leftover.
Question 5: In the reaction N₂ + 3H₂ → 2NH₃, if 1 mol N₂ and 2 mol H₂ are mixed, which is the limiting reagent?
- H₂ (Correct answer)
- N₂
- NH₃
- Both are limiting
Correct answer: H₂
1 mol N₂ requires 3 mol H₂, but only 2 mol H₂ is available, so H₂ is the limiting reagent.
Question 6: Which statement best describes the relationship between the limiting reagent and the theoretical yield?
- The limiting reagent determines the maximum theoretical yield of product (Correct answer)
- The excess reagent determines the theoretical yield
- Theoretical yield is unrelated to any single reactant
- Theoretical yield depends on the product's molar mass alone
Correct answer: The limiting reagent determines the maximum theoretical yield of product
Because the limiting reagent is fully consumed first, all product calculations are based on the amount of limiting reagent present.
RPAL Limiting Reagents 1
What is a limiting reagent in a chemical reaction?