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Lesson · Grade 11 · Chemistry

Molar mass: adding up atomic masses

  • Grade 11
  • Chemistry
  • 2 min 15 s
  • British Columbia
  • BC11-SC-20

One table builds up step by step: each element's tile from the periodic table, times its number of atoms (the subscript moves into place), then the masses added to give the molar mass in g/mol. It works H₂O = 2 × 1.01 + 16.00 = 18.02 g/mol, then CO₂ = 44.01 g/mol and NaCl = 58.44 g/mol, with notes on what a mole and molar mass are, and ends on a one-line recap.

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In one sentence

The question this lesson answers

What is the mass of one mole of water, H2O?

A mole is a fixed number of particles: about 6.02 times ten to the power of 23 of them.

Transcript

What the lesson says

What is the mass of one mole of water, H2O?

A mole is a fixed number of particles: about 6.02 times ten to the power of 23 of them.

Molar mass is the mass of one mole of a substance, measured in grams per mole.

The periodic table gives each element's atomic mass. Read in grams per mole, it is the mass of one mole of its atoms. Hydrogen is 1.01 and oxygen is 16.00.

The small 2 in H2O is a subscript. It counts the atoms of hydrogen, the element just before it.

So a water molecule has two hydrogen atoms. Oxygen has no subscript, so there is one oxygen atom.

Multiply each atomic mass by its number of atoms: 2 times 1.01 is 2.02, and 1 times 16.00 is 16.00.

Then add: 2.02 plus 16.00 gives 18.02 grams per mole, the molar mass of water.

So one mole of water molecules has a mass of 18.02 grams.

Now carbon dioxide, CO2. Carbon has no subscript, so there is one carbon atom, and the 2 means two oxygen atoms.

12.01 plus 2 times 16.00, which is 32.00, gives 44.01 grams per mole, the molar mass of carbon dioxide.

Table salt, sodium chloride, NaCl, has no subscripts, so we count one sodium and one chlorine.

22.99 plus 35.45 gives 58.44 grams per mole, the molar mass of sodium chloride.

To find a molar mass, multiply each atomic mass by its number of atoms, then add them up.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, The mole and stoichiometry: the moleBC11-SC-20

Previous: The mole: counting particles by weighingNext: The factor-label method: let the units guide you

The outcomes and strands follow the British Columbia curriculum.

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