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Lesson · Grade 11 · Chemistry

Grams to Moles to Particles

  • Grade 11
  • Chemistry
  • 1 min 57 s
  • British Columbia
  • BC11-SC-23

A road map of three boxes (grams, moles, particles), linked by molar mass and Avogadro's number, builds up while 36.0 g of water is converted step by step: to 2.00 mol, then 1.20 × 10²⁴ molecules, then 3.61 × 10²⁴ atoms. Each conversion factor is written as a fraction and the cancelling units are struck through, so students see how dimensional analysis picks each factor and why the answers keep three significant figures.

Concept: Converting grams to moles to particles

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In one sentence

The question this lesson answers

How many water molecules are in 36.0 grams of water?

We can't count molecules directly, so we go from grams, to moles, to particles.

Transcript

What the lesson says

How many water molecules are in 36.0 grams of water?

We can't count molecules directly, so we go from grams, to moles, to particles.

A mole is a counting unit: 6.02 times 10 to the power of 23 particles, called Avogadro's number.

Molar mass is the mass of one mole: for water, 18.02 grams per mole.

Start with 36.0 grams and multiply by 1 mole over 18.02 grams, a fraction equal to one.

Grams cancel, leaving 2.00 moles. Choosing factors so units cancel is called dimensional analysis.

36.0 has three significant figures, the meaningful digits, so our answers keep three.

Next, multiply 2.00 moles by 6.02 times 10 to the power of 23 molecules over 1 mole.

Moles cancel, giving 1.204 times 10 to the power of 24 molecules.

To three significant figures, that is 1.20 times 10 to the power of 24 molecules.

Each water molecule has exactly three atoms: one oxygen and two hydrogen.

Start from 1.204, the value before rounding, and multiply by 3 atoms over 1 molecule.

Molecules cancel, giving 3.612, which rounds to 3.61 times 10 to the power of 24 atoms.

The map works both ways: pick each factor so the units cancel, from grams to particles or back.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, The mole and stoichiometry: dimensional analysisBC11-SC-23

Previous: Significant Figures: How Precise Is Your Answer?Next: Stoichiometry: Using Mole Ratios to Predict Amounts

The outcomes and strands follow the British Columbia curriculum.

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