Reactants, Products and Leftovers (RPAL) Chemistry Assessment — Questions and Answers

Question 1: In CH₄ + 2O₂ → CO₂ + 2H₂O, if 3 mol CH₄ and 8 mol O₂ react, how many moles of O₂ remain?

2 mol (Correct answer)
0 mol
1 mol
3 mol

Correct answer: 2 mol

3 mol CH₄ requires 6 mol O₂; 8 − 6 = 2 mol O₂ remain as excess.

Question 2: Which expression correctly rearranges percent yield to find actual yield?

Actual yield = theoretical yield / (percent yield × 100)
Actual yield = theoretical yield − percent yield
Actual yield = percent yield + theoretical yield
Actual yield = (percent yield / 100) × theoretical yield (Correct answer)

Correct answer: Actual yield = (percent yield / 100) × theoretical yield

Rearranging (actual / theoretical) × 100 = % yield gives actual yield = (% yield / 100) × theoretical yield.

Question 3: How many moles of excess reagent remain after the reaction in the previous question (2 mol Fe + 3 mol S → FeS)?

0 mol — both are consumed
1 mol Fe
2 mol S
1 mol S (Correct answer)

Correct answer: 1 mol S

Fe is limiting (2 mol), so 2 mol S is consumed, leaving 3 − 2 = 1 mol S unreacted.

Question 4: If 8 g of methane (CH₄) burns completely and produces 22 g of CO₂ and some water vapor, how much water vapor was produced?

18 g (Correct answer)
36 g
9 g
14 g

Correct answer: 18 g

CH₄ + 2O₂ → CO₂ + 2H₂O; 8 g CH₄ uses 32 g O₂ for 40 g total reactants; 40 g − 22 g CO₂ = 18 g H₂O.

Question 5: Which of the following is NOT a step in identifying the limiting reagent?

Converting mass to moles for each reactant
Calculating the percent yield of the reaction (Correct answer)
Dividing moles by stoichiometric coefficients
Comparing the resulting ratios

Correct answer: Calculating the percent yield of the reaction

Percent yield is calculated after identifying the limiting reagent; it is not part of the identification process itself.

Question 6: In the reaction 2H₂ + O₂ → 2H₂O, if you have 4 mol H₂ and 1 mol O₂, which is the limiting reagent?

Neither — they are in perfect ratio
O₂ (Correct answer)
H₂O
H₂

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 7: How can you identify the limiting reactant in a chemical reaction?

It is the reactant that is left over after the reaction
It is the reactant present in the smallest quantity
It is the reactant that causes the reaction to stop
It is the reactant that produces the least amount of product (Correct answer)

Correct answer: It is the reactant that produces the least amount of product

The limiting reactant is the one that is completely consumed first, limiting the amount of product that can be formed.

Question 8: Which of the following correctly describes the role of reactants in a chemical equation?

Reactants are placed on the right side of the equation
Reactants determine the color of the products
Reactants are always present in larger quantities than products
Reactants are the substances that react to form new products (Correct answer)

Correct answer: Reactants are the substances that react to form new products

Reactants participate in the chemical reaction, and their interaction results in the formation of products.

Question 9: If you have 10 grams of H₂ and 80 grams of O₂, which is the limiting reactant in the reaction 2H₂ + O₂ → 2H₂O?

O₂
None, they are in perfect ratio
H₂ (Correct answer)
H₂O

Correct answer: H₂

10 grams of H₂ corresponds to 5 moles (H₂ has a molar mass of 2 g/mol), and 80 grams of O₂ corresponds to 2.5 moles (O₂ has a molar mass of 32 g/mol). According to the equation, 2 moles of H₂ are required for every 1 mole of O₂, so H₂ is the limiting reactant.

Question 10: Conservation of mass requires that a chemical equation must be:

Unbalanced
Balanced (Correct answer)
Written with whole number subscripts only
Written with the same number of compounds on each side

Correct answer: Balanced

A balanced equation ensures that the number of each type of atom is equal on both sides, reflecting conservation of mass.

Question 11: In a reaction with a 1:2 stoichiometric ratio (A:B), you have 3 mol A and 5 mol B. Which is limiting?

A
Both equally
Neither
B (Correct answer)

Correct answer: B

3 mol A requires 6 mol B; only 5 mol B is available, so B is the limiting reagent.

Question 12: Using the mole ratio, if you have 4 mol H₂ reacting via 2H₂ + O₂ → 2H₂O, how many moles of O₂ are needed?

1 mol O₂
2 mol O₂ (Correct answer)
8 mol O₂
4 mol O₂

Correct answer: 2 mol O₂

The H₂:O₂ mole ratio is 2:1, so 4 mol H₂ requires 4 × (1/2) = 2 mol O₂.

Question 13: For 2KClO₃ → 2KCl + 3O₂, if 2 mol KClO₃ decomposes, what is the theoretical yield of O₂?

4 mol O₂
1 mol O₂
3 mol O₂ (Correct answer)
2 mol O₂

Correct answer: 3 mol O₂

The 2:3 ratio of KClO₃ to O₂ means 2 mol KClO₃ produces 3 mol O₂.

Question 14: 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
Both reactants were limiting
The limiting reagent is still present

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 15: Which statement about conservation of mass in nuclear reactions is correct?

Nuclear reactions create new mass
Conservation of mass does not apply to any reactions
Mass is perfectly conserved in all nuclear reactions
A small amount of mass converts to energy in nuclear reactions, so mass alone is not conserved (Correct answer)

Correct answer: A small amount of mass converts to energy in nuclear reactions, so mass alone is not conserved

In nuclear reactions, mass-energy equivalence (E=mc²) means a tiny amount of mass converts to energy, so conservation of mass applies to chemical but not nuclear reactions.

Question 16: What is a reactant in a chemical reaction?

A substance that speeds up the reaction without being consumed
A substance that is formed as a result of the reaction
A byproduct of the reaction
A substance that undergoes change during a reaction (Correct answer)

Correct answer: A substance that undergoes change during a reaction

Reactants are the starting materials in a chemical reaction that are transformed into products.

Question 17: What does stoichiometry primarily deal with in a chemical reaction?

The quantitative relationships between reactants and products (Correct answer)
The speed of the reaction
The color change in the reaction
The energy released during the reaction

Correct answer: The quantitative relationships between reactants and products

Stoichiometry involves calculating the quantities of reactants and products in a chemical reaction based on a balanced chemical equation.

Question 18: For C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, what is the mole ratio of C₃H₈ to CO₂?

1:3 (Correct answer)
1:5
3:1
5:3

Correct answer: 1:3

The coefficients are 1 for C₃H₈ and 3 for CO₂, giving a mole ratio of 1:3.

Question 19: In a decomposition reaction, CaCO₃ → CaO + CO₂, 100 g of CaCO₃ decomposes to give 56 g of CaO. What mass of CO₂ is released?

100 g
144 g
44 g (Correct answer)
56 g

Correct answer: 44 g

By conservation of mass: mass of CO₂ = 100 g − 56 g = 44 g.

Question 20: A reaction between hydrogen and chlorine produces HCl. If 2 g of H₂ reacts with 71 g of Cl₂, what mass of HCl is produced?

36.5 g
142 g
69 g
73 g (Correct answer)

Correct answer: 73 g

By conservation of mass, 2 g H₂ + 71 g Cl₂ = 73 g HCl.

Question 21: If you start a reaction with 5 moles of reactant A and 3 moles of reactant B, but only 2 moles of reactant A are required, what happens to the excess reactant A?

It is completely used up
It remains unreacted as a leftover (Correct answer)
It becomes a product
It speeds up the reaction

Correct answer: It remains unreacted as a leftover

The excess reactant A that is not required for the reaction remains unreacted as a leftover.

Question 22: For 2SO₂ + O₂ → 2SO₃, if 4 mol SO₂ and 1 mol O₂ react, how many moles of SO₃ are produced?

4 mol
3 mol
2 mol (Correct answer)
1 mol

Correct answer: 2 mol

O₂ is limiting (1 mol O₂ can react with 2 mol SO₂ per the 2:1 ratio), producing 2 mol SO₃.

Question 23: If the theoretical yield of a reaction is 50 g but only 40 g is collected, what does this indicate?

The limiting reagent was not fully consumed
The reaction produced more than expected
The reaction was less than 100% efficient — some product was lost (Correct answer)
The theoretical yield calculation was wrong

Correct answer: The reaction was less than 100% efficient — some product was lost

Collecting less than theoretical yield is normal; it indicates product loss due to side reactions, incomplete reaction, or handling errors.

Question 24: What units are used when reporting the amount of excess reagent remaining?

Only liters
Only molecules
Moles or grams (depending on the question) (Correct answer)
Only grams

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 25: What is percent yield?

The fraction of product lost during the reaction
The ratio of actual yield to theoretical yield expressed as a percentage (Correct answer)
The percentage of reactants converted to products at equilibrium
The ratio of limiting reagent to excess reagent

Correct answer: The ratio of actual yield to theoretical yield expressed as a percentage

Percent yield = (actual yield / theoretical yield) × 100%, measuring how efficient a reaction was in the lab.

Question 26: Why must an equation be balanced before using mole ratios?

Because mole ratios are derived from stoichiometric coefficients, which are only correct in a balanced equation (Correct answer)
Because balancing changes the identity of the substances
Because unbalanced equations have no products
Because mole ratios only apply to gases

Correct answer: Because mole ratios are derived from stoichiometric coefficients, which are only correct in a balanced equation

Stoichiometric coefficients in a balanced equation reflect the law of conservation of mass; incorrect coefficients give wrong mole ratios.

Question 27: In the chemical equation 2H₂ + O₂ → 2H₂O, which are the reactants?

O₂ and H₂O
H₂
H₂O
H₂ and O₂ (Correct answer)

Correct answer: H₂ and O₂

In this equation, hydrogen (H₂) and oxygen (O₂) are the reactants that combine to form water (H₂O).

Question 28: A balanced chemical equation demonstrates conservation of mass because:

It has the same number of compounds on both sides
It has the same number of coefficients on both sides
It has the same number of molecules on both sides
It has the same number of atoms of each element on both sides (Correct answer)

Correct answer: It has the same number of atoms of each element on both sides

A balanced equation has equal numbers of each type of atom on both sides, ensuring mass is conserved since atoms are not created or destroyed.

Question 29: What does a percent yield of 95% tell a chemist?

The reaction was 5% complete
5% of the expected product was lost or not formed (Correct answer)
95% of reactants were left over
The limiting reagent was 95% consumed

Correct answer: 5% of the expected product was lost or not formed

A 95% yield means 5% of the theoretical product was lost, which is a very good result in synthetic chemistry.

Reactants, Products and Leftovers (RPAL) Chemistry Assessment

An educational assessment based on the PhET Interactive Simulation covering stoichiometry concepts including limiting reactants, excess reagents, mole ratios, and conservation of mass in chemical reactions.

Questions

30 min

Time Limit

70%

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