KnowledgeMadeFun, home

Lesson · Grade 11 · Chemistry

The mole: counting particles by weighing

  • Grade 11
  • Chemistry
  • 2 min 6 s
  • British Columbia
  • BC11-SC-19

A dozen and a mole are set side by side as counting units, with 1 mol = 6.02 × 10²³ particles (Avogadro's number), particles that can be atoms, molecules or ions, and a cloud of particles far too many to count: even at a billion a second it would take about 19 million years. Three balances then each weigh one mole (12.01 g of carbon, 22.99 g of sodium, 18.02 g of water) to introduce molar mass and to show that one mole always holds the same number of particles but has a different mass for each substance.

Anyone can watch two lessons on the website without an account. Signing in is free, no payment and no card, and opens three a day; Plus has no limit.

Loading

Share to Google Classroom

In one sentence

The question this lesson answers

Atoms are far too small and far too many to count one by one. So how do chemists count them?

Think of a dozen. A dozen is a counting unit that always means 12 things.

Transcript

What the lesson says

Atoms are far too small and far too many to count one by one. So how do chemists count them?

Think of a dozen. A dozen is a counting unit that always means 12 things.

The mole is a counting unit too, just a much bigger one. Its symbol is mol.

One mole is 6.02 times 10 to the power of 23 particles. This number is called Avogadro's number.

The particles can be atoms, molecules or ions, depending on the substance.

Counting that many one by one is impossible. Even at a billion a second, it would take about 19 million years.

But weighing is easy. Put one mole of carbon atoms on a balance, and it reads 12.01 grams.

The mass of one mole is called the molar mass. For carbon, it is 12.01 grams per mole.

So weighing out 12.01 grams of carbon gives you 6.02 times 10 to the power of 23 atoms, without counting a single one.

One mole of sodium atoms has a mass of 22.99 grams, so its molar mass is 22.99 grams per mole.

One mole of water molecules has a mass of 18.02 grams, so its molar mass is 18.02 grams per mole.

All three hold one mole, so they have the same number of particles.

But one mole of different substances does not have the same mass.

A sodium atom has almost twice the mass of a carbon atom, so a mole of sodium has almost twice the mass of a mole of carbon.

One mole of any substance has the same number of particles, 6.02 times 10 to the power of 23, but a different mass.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, The mole and stoichiometry: the moleBC11-SC-19

Next: Molar mass: adding up atomic masses

The outcomes and strands follow the British Columbia curriculum.

Keep going

More on this topic