Chemical Reactions · Grade 10

What is a mole, and why do chemists count atoms by weighing them?

A mole is a pack of a fixed number of particles — about 6.02 times ten to the twenty-third. A single atom is far too small to weigh and there are far too many to count in any real sample, but the mass of a whole pack sits comfortably on an ordinary balance. The molar mass is read straight off the periodic table, so weighing becomes counting.

Learning objectives

Why the unit is needed at all

A balanced equation says 'one methane molecule reacts with two oxygen molecules'. Nobody in a laboratory measures single molecules — they measure grams, and a spoonful of sugar holds more molecules than there are grains of sand on every beach on Earth.

The mole bridges that gap. It is not a unit of mass or of volume but of amount, exactly like a dozen — except that instead of twelve it is about 6.02 times ten to the twenty-third, a figure called Avogadro's number.

That number was not chosen arbitrarily. It was chosen so that one mole of an element weighs, in grams, precisely the number printed in the periodic table — and that is what turns the table into a measuring instrument rather than just a chart.

Molar mass

The molar mass of an element is its atomic mass in grams per mole. Carbon is 12, oxygen 16, iron about 56.

For a compound you add them up. Water, H2O, has two hydrogen atoms of mass 1 each and one oxygen of 16, so 18 g per mole. Carbon dioxide, CO2, gives 12 plus two lots of 16, which is 44.

From that come two conversions worth memorising: the number of moles is the mass divided by the molar mass, and the number of particles is the number of moles times Avogadro's number.

Why a mole of iron outweighs a mole of carbon

Both contain exactly the same number of atoms. The difference is that an iron atom is several times heavier than a carbon atom — its nucleus holds far more protons and neutrons.

So a mole of carbon weighs 12 g and a mole of iron about 56 g, with identical particle counts in each. A mole counts; it does not weigh.

Worked examples

  1. What is the molar mass of CO2?

    1. Carbon is 12
    2. Oxygen is 16, and there are two: 32
    3. 12 plus 32

    Answer: 44 g per mole

  2. How many moles are there in 36 g of water?

    1. The molar mass of water is 18 g per mole
    2. Moles = mass divided by molar mass
    3. 36 divided by 18

    Answer: 2 moles

  3. How many molecules are in 2 moles of water?

    1. Each mole holds about 6.02 times ten to the twenty-third particles
    2. Multiply by two

    Answer: About 1.2 times ten to the twenty-fourth molecules

Common mistakes

Treating the mole as a unit of mass
A mole is an amount of particles, like a dozen. A mole of feathers and a mole of lead hold the same number of particles and weigh nothing like the same.
Dividing by the molar mass the wrong way round
Mass divided by molar mass gives moles. If the answer is larger than the mass in grams, the division was done upside down.
Forgetting an atom that appears more than once
CO2 has two oxygen atoms, so 32 and not 16. That is the slip that turns 44 into 28 and takes every calculation built on it down with it.

What to remember

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