RPAL Excess Reagents 2 — Questions and Answers
Question 1: In N₂ + 3H₂ → 2NH₃, you start with 2 mol N₂ and 3 mol H₂. After the reaction, how many moles of excess reagent remain?
- N₂; 1 mol remains (Correct answer)
- H₂; 1 mol remains
- N₂; 0.5 mol remains
- H₂; 3 mol remain
Correct answer: N₂; 1 mol remains
H₂ is limiting (3 mol H₂ can only react with 1 mol N₂), leaving 2 − 1 = 1 mol N₂ as excess.
Question 2: What units are used when reporting the amount of excess reagent remaining?
- Moles or grams (depending on the question) (Correct answer)
- Only grams
- Only liters
- Only molecules
Correct answer: Moles or grams (depending on the question)
Excess reagent can be reported in moles or converted to grams using molar mass, depending on what the problem asks for.
Question 3: Why is it useful to calculate the amount of excess reagent remaining?
- To know what leftover materials are present after the reaction (Correct answer)
- To calculate the activation energy
- To determine the rate of the reaction
- To find the equilibrium constant
Correct answer: To know what leftover materials are present after the reaction
Knowing the excess reagent leftover helps in planning reactions, recycling materials, and understanding reaction completeness.
Question 4: For 2Fe + 3Cl₂ → 2FeCl₃, if 4 mol Fe and 3 mol Cl₂ react, how many moles of excess Fe remain?
- 2 mol Fe (Correct answer)
- 1 mol Fe
- 4 mol Fe
- 0 mol Fe
Correct answer: 2 mol Fe
3 mol Cl₂ can react with only 2 mol Fe (from the 2:3 ratio), so 4 − 2 = 2 mol Fe remain as excess.
Question 5: A student reports that after a reaction, 0.5 mol of the excess reagent remains. What does this tell you about the reaction?
- The reaction went to completion with the limiting reagent fully consumed (Correct answer)
- The reaction did not occur
- The limiting reagent is still present
- Both reactants were limiting
Correct answer: The reaction went to completion with the limiting reagent fully consumed
Leftover excess reagent confirms the reaction went to completion and the limiting reagent was fully used up.
Question 6: If both reactants are used up completely in a reaction, what can you conclude?
- They were in a perfect stoichiometric ratio with no excess (Correct answer)
- One reactant must still be limiting
- The reaction did not go to completion
- The product yield must be 100%
Correct answer: They were in a perfect stoichiometric ratio with no excess
When both reactants are fully consumed, they were present in exactly the stoichiometric ratio required — neither is in excess.
RPAL Excess Reagents 2
In N₂ + 3H₂ → 2NH₃, you start with 2 mol N₂ and 3 mol H₂.
After the reaction, how many moles of excess reagent remain?