Reactions, Moles & Composition — Problem set, Grade 11

Account for atoms, charge, particle counts, and measured composition.

Review the key ideas

Types of Reactions

Reaction patterns are useful classifications that can overlap. Product predictions need chemical evidence and conditions in addition to a balanced formula pattern; conserve atoms, charge and species-state information.

Balancing Chemical Equations

Because atoms are conserved, balance an equation by changing only coefficients (never subscripts) until every element has equal atoms on both sides, using the smallest whole-number ratio.

The Mole & Molar Mass

One mole contains exactly 6.02214076×10²³ specified entities. Molar mass connects amount with mass, and formula composition connects molecule/formula-unit counts with constituent atom or ion counts.

Percent Composition, Empirical & Molecular Formulas

Convert elemental mass to mole ratios; use independent molar mass to find molecular subscripts.

Aqueous Reactions & Net Ionic Equations

Net ionic equations remove spectators and conserve atoms and charge.

Problem 1

Classify this reaction: CaCO₃ → CaO + CO₂.
Answer and explanation

Decomposition

A single compound (calcium carbonate) breaks apart into two simpler substances, matching AB → A + B. That is a decomposition reaction.

Problem 2

Zinc metal is placed in hydrochloric acid (HCl). Identify the reaction type and predict the products.
Answer and explanation

Single replacement; the products are zinc chloride (ZnCl₂) and hydrogen gas (H₂): Zn + 2HCl → ZnCl₂ + H₂.

A lone element reacting with a compound is a single-replacement reaction (A + BC → AC + B). Zinc is more reactive than hydrogen, so it displaces the hydrogen, bonding with chlorine to form ZnCl₂ and releasing H₂ gas. Balanced: Zn + 2HCl → ZnCl₂ + H₂.

Problem 3

Balance the equation: ___H₂ + ___O₂ → ___H₂O.
Answer and explanation

2H₂ + O₂ → 2H₂O

Place a 2 in front of H₂O to balance oxygen (2 O on each side), then a 2 in front of H₂ to balance hydrogen (4 H on each side): 2H₂ + O₂ → 2H₂O.

Problem 4

Balance the combustion of propane: ___C₃H₈ + ___O₂ → ___CO₂ + ___H₂O.
Answer and explanation

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Balance carbon: 3 C require 3 CO₂. Balance hydrogen: 8 H require 4 H₂O. Count oxygen on the right: (3 × 2) + (4 × 1) = 10, so 5 O₂ are needed on the left. Final: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Problem 5

Balance the decomposition of potassium chlorate: ___KClO₃ → ___KCl + ___O₂.
Answer and explanation

2KClO₃ → 2KCl + 3O₂

Potassium and chlorine already pair up as KCl, so the challenge is oxygen: KClO₃ has 3 O per unit and O₂ comes in 2 s. Use 2 KClO₃ (6 O) and 3 O₂ (6 O), which then needs 2 KCl. Recount: K 2 = 2, Cl 2 = 2, O 6 = 6. Balanced: 2KClO₃ → 2KCl + 3O₂.

Problem 6

Calculate the molar mass of water, H₂O.
Answer and explanation

18.02 g/mol

Add the atomic masses: 2(1.01) + 16.00 = 2.02 + 16.00 = 18.02 g/mol.

Problem 7

How many moles are in 36.04 g of water? (Molar mass H₂O = 18.02 g/mol.)
Answer and explanation

2.00 mol

moles = mass ÷ molar mass = 36.04 g ÷ 18.02 g/mol = 2.00 mol.

Problem 8

Find the molar mass of glucose, C₆H₁₂O₆, then determine how many moles are in 90.09 g of it.
Answer and explanation

180.18 g/mol; 0.500 mol

Molar mass = 6(12.01) + 12(1.01) + 6(16.00) = 72.06 + 12.12 + 96.00 = 180.18 g/mol. Then moles = 90.09 g ÷ 180.18 g/mol = 0.500 mol.

Problem 9

What evidence supports a precipitation reaction?
Answer and explanation

A newly formed solid plus a balanced net ionic model; exclude existing suspended material as an alternative.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 10

Chapter review: A student writes an exchange equation for two clear ionic solutions and predicts a solid solely because the equation is balanced. Explain the missing evidence and give two quantities that still must balance.
Answer and explanation

Balance atoms of each element and total charge. A precipitate prediction also needs appropriate solubility/concentration evidence under the conditions; balanced pattern matching alone does not establish a separate solid phase.

Balance atoms of each element and total charge. A precipitate prediction also needs appropriate solubility/concentration evidence under the conditions; balanced pattern matching alone does not establish a separate solid phase.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

Balance C₃H₈+O₂→CO₂+H₂O.
Answer and explanation

C₃H₈+5O₂→3CO₂+4H₂O; three C, eight H, and ten O are conserved.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 12

Chapter review: Balance C₃H₈ + O₂ → CO₂ + H₂O using the smallest whole-number coefficients, then show the atom counts.
Answer and explanation

C₃H₈ +5O₂ →3CO₂ +4H₂O. Counts are C:3, H:8, O:10 on each side. Formula subscripts remain unchanged.

C₃H₈ +5O₂ →3CO₂ +4H₂O. Counts are C:3, H:8, O:10 on each side. Formula subscripts remain unchanged.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 13

Using H=1 and O=16, how many moles are in 36 g water?
Answer and explanation

Molar mass 18 g/mol, so 36/18=2 mol; identify molecules as the counted entities.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 14

Chapter review: Using Mₘ(CO₂)=44.0 g/mol and Nₐ≈6.022×10²³ mol⁻¹, find moles, molecules and moles of oxygen atoms in 22.0 g CO₂.
Answer and explanation

n=22.0/44.0=0.500 mol CO₂. Molecules≈3.01×10²³. Each has two O atoms, giving 1.00 mol O atoms. Keep the entity labels through every conversion.

n=22.0/44.0=0.500 mol CO₂. Molecules≈3.01×10²³. Each has two O atoms, giving 1.00 mol O atoms. Keep the entity labels through every conversion.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

A sample has 2 mol H and 1 mol O. Give empirical formula and its limitation.
Answer and explanation

H₂O is the simplest ratio; composition alone does not establish molecular size or bonding.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 16

Empirical mass is 46 g/mol and molecular mass 92. Find multiplier.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

2

92/46 = 2.

Problem 17

Chapter review: A selected oxide sample has 70.0 g Fe and 30.0 g O. Use Fe=56.0 and O=16.0 g/mol to find the simplest formula. State one assumption.
Answer and explanation

Amounts are 1.25 mol Fe and 1.875 mol O, ratio 1:1.5, hence Fe₂O₃. Assume the analyzed sample is one oxide accounted for by those elements and the input precision supports that ratio.

Amounts are 1.25 mol Fe and 1.875 mol O, ratio 1:1.5, hence Fe₂O₃. Assume the analyzed sample is one oxide accounted for by those elements and the input precision supports that ratio.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 18

Write a net ionic equation for Ca²⁺ and CO₃²⁻ forming a solid.
Answer and explanation

Ca²⁺(aq)+CO₃²⁻(aq)→CaCO₃(s); one of each balances atoms and charge.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 19

Why is swapping ions not sufficient evidence of reaction?
Answer and explanation

If all species remain unchanged dissolved ions, no net chemical transformation is represented.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 20

Chapter review: Write the net ionic equation when suitable aqueous BaCl₂ and Na₂SO₄ produce BaSO₄(s). Name spectators and verify charge. This is a supplied-species exercise, not a home experiment.
Answer and explanation

Ba²⁺(aq)+SO₄²⁻(aq)→BaSO₄(s). Na⁺ and Cl⁻ are spectators in the selected account. One Ba and one sulfate group are conserved and total charge is zero on each side.

Ba²⁺(aq)+SO₄²⁻(aq)→BaSO₄(s). Na⁺ and Cl⁻ are spectators in the selected account. One Ba and one sulfate group are conserved and total charge is zero on each side.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
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