RPAL Excess Reagents 1 — Questions and Answers
Question 1: What is an excess reagent?
- A reactant present in greater amount than needed to react with the limiting reagent (Correct answer)
- The reactant that is fully consumed in the reaction
- Any product formed in a reaction
- A catalyst added to speed up the reaction
Correct answer: A reactant present in greater amount than needed to react with the limiting reagent
An excess reagent is any reactant that remains after the limiting reagent is completely consumed.
Question 2: How is the amount of excess reagent remaining calculated?
- Subtract the moles consumed (based on limiting reagent) from the initial moles (Correct answer)
- Divide the initial moles by the molar mass
- Add the moles of product formed to the initial moles
- Multiply the limiting reagent moles by Avogadro's number
Correct answer: Subtract the moles consumed (based on limiting reagent) from the initial moles
Using stoichiometry, calculate how much excess reagent reacted with the limiting reagent, then subtract from the starting amount.
Question 3: In 2H₂ + O₂ → 2H₂O, if 6 mol H₂ and 2 mol O₂ react, which is the excess reagent and how much remains?
- Hâ‚‚; 2 mol remain (Correct answer)
- Oâ‚‚; 2 mol remain
- Hâ‚‚; 1 mol remains
- Oâ‚‚; 1 mol remains
Correct answer: Hâ‚‚; 2 mol remain
2 mol O₂ consumes 4 mol H₂ (from the 2:1 ratio), leaving 6 − 4 = 2 mol H₂ unreacted.
Question 4: If you increase the amount of excess reagent in a reaction, what happens to the amount of product formed?
- Nothing — product amount depends only on the limiting reagent (Correct answer)
- Product increases proportionally
- Product decreases because the excess interferes
- Product doubles for every mole of excess added
Correct answer: Nothing — product amount depends only on the limiting reagent
Since the limiting reagent controls product formation, adding more excess reagent does not change the yield.
Question 5: Which of the following best describes 'leftover' in the RPAL context?
- The unreacted excess reagent remaining after the reaction stops (Correct answer)
- The amount of limiting reagent consumed
- The theoretical yield of the product
- The activation energy of the reaction
Correct answer: The unreacted excess reagent remaining after the reaction stops
In RPAL problems, 'leftover' specifically refers to the amount of excess reagent that does not react because the limiting reagent runs out.
Question 6: For the reaction C + O₂ → CO₂, if 3 mol C and 5 mol O₂ are combined, what is the excess reagent?
- Oâ‚‚ (Correct answer)
- C
- COâ‚‚
- There is no excess reagent
Correct answer: Oâ‚‚
The 1:1 ratio means 3 mol C consumes only 3 mol O₂, leaving 5 − 3 = 2 mol O₂ as excess.
RPAL Excess Reagents 1
What is an excess reagent?