Counting by weighing.

Step 1 of 5

Counting by weighing

A hardware shop does not count screws one by one: it weighs them. If one screw weighs 2 g, a 200 g bag holds a hundred screws. Chemists do the same with atoms, only on an altogether different scale.

The unit for counting particles is the mole. One mole is a fixed number of particles, the Avogadro constant:

$N_A = 6.022 \times 10^{23}$$N_A =$$6.022 \times 10^{23}$⁠

The number was chosen so that a mole of carbon-12 atoms weighs exactly 12 g. So a mole of any substance weighs its relative formula mass in grams. The Scene shows 12 g of carbon, with the readings hiding the number of atoms. How many atoms are there?

Step 2 of 5

The same number, a different mass

The balance holds 27 g of aluminium granules: one mole, $6.02 \times 10^{23}$⁠ atoms. Mass is locked. Predict what happens when the aluminium is swapped for one mole of gold, exactly the same number of atoms.

Step 3 of 5

Molar mass

The mass of one mole of a substance is its molar mass, $M$⁠, measured in grams per mole, g/mol. Its number equals the relative formula mass: water’s $M_r$⁠ is $18.02$⁠, so the molar mass of water is 18.02 g/mol. The only difference is the unit: $M_r$⁠ compares masses, and $M$⁠ says how many grams a mole weighs.

The Scene shows ethanol, C₂H₅OH, the alcohol in drinks. Note that it has six hydrogens, five in the C₂H₅ group and one more in OH.

Step 4 of 5

Weighing out a mole

Now an exact amount of atoms can be weighed out. The Scene shows iron, with the readings showing only the mass. Use Mass to weigh out exactly one mole of iron atoms.

Step 5 of 5

The same mass, a different number

The other side of the same coin: equal masses of different substances hold different numbers of particles. The Scene shows 12 g of carbon, one mole. Change Substance and see how many atoms the same 12 g of iron or gold holds.