The Mole & Molar Mass — Practice · Grade 11 Science

Chapter: Reactions, Moles & Composition

What this practice covers

By the end of The Mole & Molar Mass you should be able to:

10 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: mole (mol), Avogadro's number, molar mass, atomic mass, representative particle.

Problem 1

Calculate the molar mass of carbon dioxide, CO₂. (C = 12.01, O = 16.00 g/mol.)
Answer and explanation

44.01 g/mol

Molar mass = 12.01 + 2(16.00) = 12.01 + 32.00 = 44.01 g/mol.

Problem 2

Avogadro's number — the number of representative particles in one mole — is approximately…
  1. 6.022 × 10²³
  2. 3.14 × 10⁰
  3. 1.602 × 10⁻¹⁹
  4. 9.81 × 10⁰
Answer and explanation

6.022 × 10²³

One mole is defined as 6.022 × 10²³ particles.

Problem 3

How many moles are in 88.02 g of carbon dioxide? (Molar mass CO₂ = 44.01 g/mol.)
Answer and explanation

2 mol

moles = mass ÷ molar mass = 88.02 g ÷ 44.01 g/mol = 2.00 mol.

Problem 4

What is the mass of 3.00 mol of water? (Molar mass H₂O = 18.02 g/mol.)
Answer and explanation

54.06 g

mass = moles × molar mass = 3.00 mol × 18.02 g/mol = 54.06 g.

Problem 5

How many molecules are in 2.00 mol of carbon dioxide? (Avogadro's number = 6.022 × 10²³ per mole.)
Answer and explanation

1.2044e+24 molecules

molecules = moles × Avogadro's number = 2.00 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules.

Problem 6

Calculate the molar mass of glucose, C₆H₁₂O₆. (C = 12.01, H = 1.01, O = 16.00 g/mol.)
Answer and explanation

180.18 g/mol

6(12.01) + 12(1.01) + 6(16.00) = 72.06 + 12.12 + 96.00 = 180.18 g/mol.

Problem 7

Find moles in 11.0 g CO₂ using 44.0 g/mol. Show the reasoning, not just the conclusion.
Hints

Identify the requested entity before applying the mole conversion.

Answer and explanation

11.0/44.0=0.250 mol.

Problem 8

How many oxygen atoms are in 0.50 mol O₂ using Nₐ=6.02×10²³?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

0.50 mol molecules×2=1.0 mol O atoms, or 6.02×10²³ atoms.

Problem 9

Units connect measured mass to amount and entity countUnits connect measured mass to amount and entity count36.0 g H₂O2.00 mol H₂O÷ 18.0 g/mol2.00 mol × 6.022×10²³ molecules/mol= 1.20×10²⁴ H₂O molecules
Original rounded conversion using a supplied water molar mass; entity count precision reflects the measured mass and chosen school constants.
Interpret the supplied visual. What would be wrong with multiplying 36.0 g directly by a number in molecules/mol?
Answer and explanation

The grams would not cancel the mole denominator. First convert mass to moles with the substance’s molar mass.

Convert mass to amount using n=m/Mₘ, then multiply amount by the Avogadro constant to obtain specified-entity count. Each conversion factor must cancel the previous unit and retain the entity name. The selected 18.0 g/mol water value is a rounded school input; using it consistently gives 2.00 mol from 36.0 g and approximately 1.20×10²⁴ water molecules. Mole amount is not itself a mass or volume, so do not skip the chemical composition step.

The grams would not cancel the mole denominator. First convert mass to moles with the substance’s molar mass.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

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.

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