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

Question 1: Which statement about conservation of mass in nuclear reactions is correct?

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)
Nuclear reactions create new mass
Conservation of mass does not apply to any reactions

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 2: A reaction produces 18 g of water and 44 g of CO₂ from methane and oxygen. If 16 g of methane was used, how much oxygen was consumed?

46 g (Correct answer)
32 g
62 g
26 g

Correct answer: 46 g

Total product mass = 18 + 44 = 62 g; mass of O₂ = 62 g − 16 g (CH₄) = 46 g.

Question 3: In the reaction N₂ + 3H₂ → 2NH₃, if 28 g of N₂ reacts with 6 g of H₂, what mass of NH₃ is produced?

34 g (Correct answer)
40 g
28 g
17 g

Correct answer: 34 g

By conservation of mass, 28 g N₂ + 6 g H₂ = 34 g NH₃, since all reactants are consumed and mass is conserved.

Question 4: In the balanced equation 2H₂ + O₂ → 2H₂O, how many moles of water are produced from 3 moles of oxygen?

4 moles
6 moles (Correct answer)
3 moles
2 moles

Correct answer: 6 moles

The balanced equation shows that 1 mole of O₂ produces 2 moles of H₂O. Therefore, 3 moles of O₂ will produce 6 moles of H₂O.

Question 5: 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 are the substances that react to form new products (Correct answer)
Reactants are always present in larger quantities than products
Reactants determine the color of the products

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 6: What is an excess reagent?

The reactant that is fully consumed in the reaction
A reactant present in greater amount than needed to react with the limiting reagent (Correct answer)
A catalyst added to speed up the reaction
Any product formed in a 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 7: In the reaction 2H₂ + O₂ → 2H₂O, if O₂ is the limiting reagent with 2 mol, what is the theoretical yield of H₂O?

2 mol H₂O
1 mol H₂O
4 mol H₂O (Correct answer)
3 mol H₂O

Correct answer: 4 mol H₂O

The 1:2 ratio of O₂ to H₂O means 2 mol O₂ produces 2 × 2 = 4 mol H₂O.

Question 8: Why is percent yield almost always less than 100%?

Because theoretical yield is always overestimated
Because Avogadro's number introduces rounding errors
Because the limiting reagent is never fully identified correctly
Because of side reactions, product loss during purification, and incomplete reactions (Correct answer)

Correct answer: Because of side reactions, product loss during purification, and incomplete reactions

Real reactions lose product through side reactions, handling losses, and incomplete conversion, making 100% yield essentially unattainable.

Question 9: Antoine Lavoisier established the law of conservation of mass in the 18th century primarily by:

Observing color changes during reactions
Measuring temperature changes in reactions
Carefully measuring masses before and after reactions in sealed containers (Correct answer)
Counting the number of molecules in reactions

Correct answer: Carefully measuring masses before and after reactions in sealed containers

Lavoisier used precise mass measurements in sealed containers to show that total mass is unchanged before and after chemical reactions.

Question 10: Why does an open container experiment sometimes appear to violate conservation of mass when a substance burns?

The scale measurement is always wrong
Mass is destroyed during combustion
The reaction creates new matter
Gas-phase products like CO₂ escape into the atmosphere (Correct answer)

Correct answer: Gas-phase products like CO₂ escape into the atmosphere

In an open system, gaseous products like CO₂ and water vapor escape, making the remaining solid or liquid mass appear to decrease, but total mass is still conserved.

Question 11: 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 mole ratios only apply to gases
Because unbalanced equations have no products

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

4 mol O₂
2 mol O₂ (Correct answer)
1 mol O₂
8 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: In the reaction N₂ + 3H₂ → 2NH₃, if 1 mol N₂ and 2 mol H₂ are mixed, which is the limiting reagent?

Both are limiting
N₂
H₂ (Correct answer)
NH₃

Correct answer: H₂

1 mol N₂ requires 3 mol H₂, but only 2 mol H₂ is available, so H₂ is the limiting reagent.

Question 14: 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 becomes a product
It is completely used up
It remains unreacted as a leftover (Correct answer)
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 15: A mole ratio can be written as either A/B or B/A. Which form do you use?

The form that cancels the unit you want to eliminate and gives the unit you want (Correct answer)
Always B/A regardless of context
Always A/B regardless of context
The form that gives the largest numerical answer

Correct answer: The form that cancels the unit you want to eliminate and gives the unit you want

Choose the form of the mole ratio that cancels the given unit and produces the desired unit in dimensional analysis.

Question 16: For 4Fe + 3O₂ → 2Fe₂O₃, if 2 mol O₂ is the limiting reagent, what is the theoretical yield of Fe₂O₃?

4/3 mol ≈ 1.33 mol (Correct answer)
3 mol
2 mol
4 mol

Correct answer: 4/3 mol ≈ 1.33 mol

The 3:2 ratio of O₂ to Fe₂O₃ means 2 mol O₂ produces 2 × (2/3) = 4/3 ≈ 1.33 mol Fe₂O₃.

Question 17: If the theoretical yield of a product is 0.25 mol and its molar mass is 180 g/mol, what is the theoretical yield in grams?

720 g
45 g (Correct answer)
0.0014 g
4.5 g

Correct answer: 45 g

0.25 mol × 180 g/mol = 45 g of product.

Question 18: In 2KClO₃ → 2KCl + 3O₂, how many moles of O₂ are produced from 4 mol KClO₃?

6 mol O₂ (Correct answer)
8 mol O₂
4 mol O₂
3 mol O₂

Correct answer: 6 mol O₂

The KClO₃:O₂ ratio is 2:3, so 4 mol KClO₃ produces 4 × (3/2) = 6 mol O₂.

Question 19: Which scenario correctly applies conservation of mass?

15 g reactant → 14 g product
10 g reactant → 8 g product + 3 g gas
10 g reactant → 10 g product
12 g reactant → 10 g product + 2 g gas (Correct answer)

Correct answer: 12 g reactant → 10 g product + 2 g gas

12 g reactant producing 10 g product + 2 g gas satisfies conservation of mass because 10 + 2 = 12 g total product mass.

Question 20: If you increase the amount of excess reagent in a reaction, what happens to the amount of product formed?

Product decreases because the excess interferes
Nothing — product amount depends only on the limiting reagent (Correct answer)
Product increases proportionally
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 21: If both reactants are used up completely in a reaction, what can you conclude?

The product yield must be 100%
The reaction did not go to completion
One reactant must still be limiting
They were in a perfect stoichiometric ratio with no excess (Correct answer)

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.

Question 22: In the reaction 2H₂ + O₂ → 2H₂O, if you have 4 mol H₂ and 1 mol O₂, which is the limiting reagent?

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

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 23: A percent yield greater than 100% indicates what?

An exceptionally efficient reaction
An error — possibly impure product or incorrect theoretical yield calculation (Correct answer)
The reaction produced extra atoms from the environment
The actual yield was calculated incorrectly and is actually valid

Correct answer: An error — possibly impure product or incorrect theoretical yield calculation

A percent yield over 100% is physically impossible and signals experimental error such as product contamination or a miscalculated theoretical yield.

Question 24: For N₂ + 3H₂ → 2NH₃, what is the mole ratio of H₂ to NH₃?

3:2 (Correct answer)
1:2
3:1
2:3

Correct answer: 3:2

The coefficients are 3 for H₂ and 2 for NH₃, giving a 3:2 mole ratio.

Question 25: For 2SO₂ + O₂ → 2SO₃, if 4 mol SO₂ and 1 mol O₂ react, how many moles of SO₃ are produced?

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

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 26: If a reaction has multiple steps, each with 90% yield, what is the overall yield after two steps?

180%
81% (Correct answer)
90%
45%

Correct answer: 81%

Overall yield = 90% × 90% = 0.9 × 0.9 = 0.81 = 81%; yields multiply across steps.

Question 27: In a chemical reaction, 25 g of substance X and 15 g of substance Y react to produce substance Z and 5 g of a gas. What is the mass of Z?

45 g
35 g (Correct answer)
25 g
15 g

Correct answer: 35 g

By conservation of mass: mass of Z = (25 + 15) − 5 = 35 g.

Question 28: What is a reactant in a chemical reaction?

A substance that undergoes change during a reaction (Correct answer)
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

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 29: For a reaction producing aspirin, if the theoretical yield is 3.00 g and 2.64 g is collected, what is the percent yield?

12%
114%
88% (Correct answer)
88.8 g

Correct answer: 88%

(2.64 / 3.00) × 100% = 88% yield.

Question 30: What is theoretical yield?

The minimum amount of product that can form
The maximum amount of product that can be formed from the limiting reagent (Correct answer)
The amount of excess reagent that remains
The actual amount of product collected in the lab

Correct answer: The maximum amount of product that can be formed from the limiting reagent

Theoretical yield is the calculated maximum product assuming all of the limiting reagent is converted to product with no losses.

Question 31: 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?

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

Correct answer: 73 g

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

Question 32: How can you identify the limiting reactant in a chemical reaction?

It is the reactant that causes the reaction to stop
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 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 33: For the reaction C + O₂ → CO₂, if 3 mol C and 5 mol O₂ are combined, what is the excess reagent?

O₂ (Correct answer)
C
CO₂
There is no excess reagent

Correct answer: O₂

The 1:1 ratio means 3 mol C consumes only 3 mol O₂, leaving 5 − 3 = 2 mol O₂ as excess.

Question 34: A student calculates a theoretical yield of 8.5 g for a product with a molar mass of 17 g/mol. How many moles of product does this represent?

0.05 mol
0.5 mol (Correct answer)
5.0 mol
144.5 mol

Correct answer: 0.5 mol

8.5 g ÷ 17 g/mol = 0.5 mol of product.

Question 35: In a chemical reaction, what are the substances that are initially present and undergo a transformation?

Leftovers
Catalysts
Reactants (Correct answer)
Products

Correct answer: Reactants

Reactants are the starting materials in a chemical reaction that undergo change to form products.

Question 36: In the chemical equation 2H₂ + O₂ → 2H₂O, which are the reactants?

H₂O
O₂ and H₂O
H₂
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 37: Which of the following is a common source of percent yield less than 100% in lab experiments?

Measuring mass before the reaction
Adding reactants in the correct order
Using too large a beaker
Product left on glassware during transfer (Correct answer)

Correct answer: Product left on glassware during transfer

Product sticking to glassware during transfer is a frequent mechanical loss that lowers actual yield.

Question 38: Which of the following is NOT a step in identifying the limiting reagent?

Calculating the percent yield of the reaction (Correct answer)
Dividing moles by stoichiometric coefficients
Converting mass to moles for each reactant
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 39: For C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, what is the mole ratio of C₃H₈ to CO₂?

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

Correct answer: 1:3

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

Question 40: Conservation of mass requires that a chemical equation must be:

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

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 41: What assumption underlies the theoretical yield calculation?

The excess reagent contributes equally to product formation
The reaction proceeds to completion with 100% conversion of limiting reagent to product (Correct answer)
Some limiting reagent always remains unreacted
The reaction temperature affects the amount of product

Correct answer: The reaction proceeds to completion with 100% conversion of limiting reagent to product

Theoretical yield assumes all of the limiting reagent is converted to product with no side reactions or losses.

Question 42: Which law states that matter is neither created nor destroyed in a chemical reaction?

Law of Multiple Proportions
Law of Conservation of Energy
Law of Definite Proportions
Law of Conservation of Mass (Correct answer)

Correct answer: Law of Conservation of Mass

The Law of Conservation of Mass states that the total mass of reactants equals the total mass of products in a chemical reaction.

Question 43: Why is the theoretical yield considered an 'ideal' value?

Because only ideal solutions can achieve this yield
Because it assumes perfect conditions with no product lost to side reactions or handling (Correct answer)
Because it is calculated using ideal gas conditions
Because it is always larger than the limiting reagent mass

Correct answer: Because it assumes perfect conditions with no product lost to side reactions or handling

Theoretical yield represents a best-case scenario that rarely occurs in practice due to side reactions, incomplete reactions, and product losses during purification.

Question 44: Which of the following best describes 'leftover' in the RPAL context?

The theoretical yield of the product
The activation energy of the reaction
The unreacted excess reagent remaining after the reaction stops (Correct answer)
The amount of limiting reagent consumed

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 45: Why are mole ratios central to all stoichiometry calculations?

They determine the reaction rate
They link the quantities of all substances in a reaction through the balanced equation (Correct answer)
They define the physical states of reactants and products
They predict whether a reaction is exothermic or endothermic

Correct answer: They link the quantities of all substances in a reaction through the balanced equation

Mole ratios are the bridge between moles of any two species in a reaction, enabling all quantitative predictions in stoichiometry.

Question 46: What quantity can be calculated directly from the moles of the limiting reagent?

Activation energy of the reaction
Moles of excess reagent remaining
Moles of product formed (Correct answer)
Enthalpy change of the reaction

Correct answer: Moles of product formed

Using the mole ratio from the balanced equation, the moles of limiting reagent directly yield the moles of product formed.

Question 47: What units are used when reporting the amount of excess reagent remaining?

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

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

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

Correct answer: 1 mol S

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

Question 49: When iron rusts, it gains mass. This is consistent with conservation of mass because:

The density increases, so mass increases
The iron absorbs oxygen from the air, adding to the total mass (Correct answer)
Mass is created during oxidation
Some energy converts to mass

Correct answer: The iron absorbs oxygen from the air, adding to the total mass

Rusting is Fe + O₂ → Fe₂O₃; oxygen from air combines with iron, so the iron's mass increases while the total system mass (iron + oxygen) remains constant.

Question 50: Which scenario results in the largest amount of excess reagent remaining?

Starting with a large surplus of one reactant relative to the stoichiometric requirement (Correct answer)
Starting with equal masses of both reactants
Using the minimum required amount of each reactant
Using exactly stoichiometric amounts of both reactants

Correct answer: Starting with a large surplus of one reactant relative to the stoichiometric requirement

The greater the surplus of one reactant relative to the stoichiometric need, the more excess remains after the reaction.

Question 51: A reaction has a theoretical yield of 25 g and a percent yield of 72%. What is the actual yield?

34.7 g
18 g (Correct answer)
25 g
72 g

Correct answer: 18 g

Actual yield = (72/100) × 25 g = 18 g.

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

60 min

Time Limit

70%

To Pass

Checking for saved progress...

Exam Rules

You can skip questions and return to them later
Flag questions for review before submitting
No feedback shown until you submit the entire exam
Unanswered questions count as wrong — answer everything
10 pretest questions are mixed in and don't affect your score
Timer auto-submits when time runs out
Your progress is auto-saved every 30 seconds