What is a mole in chemistry?
A mole is a counting unit: exactly 6.02214076 × 10^23 particles, a number called Avogadro's number. Chemists use it because one mole of a substance has a mass in grams equal to its formula mass, which turns weighing into counting.
Why a counting unit
Reactions happen particle by particle: two hydrogen molecules react with one oxygen molecule, not two grams with one gram. But atoms are far too small to count one at a time, so chemists count them in batches. A mole works like a dozen, only much bigger, and it was chosen so the numbers line up with mass.
One carbon-12 atom has a mass of exactly 12 atomic mass units (u), so a mole of carbon-12 atoms has a mass of 12 grams. In general, molar mass in grams per mole has the same number as the average mass of one particle in u. Water's formula mass is about 18.02 u, so its molar mass is 18.02 g/mol.
Worked example
How many water molecules are in 36.0 g of water?
- Molar mass of H2O: 2(1.008) + 16.00 = 18.02 g/mol.
- Moles: 36.0 g ÷ 18.02 g/mol = 2.00 mol.
- Molecules: 2.00 mol × 6.022 × 10^23 per mol = 1.20 × 10^24 molecules.
The same chain runs backward, and through a balanced equation. In 2 H2 + O2 → 2 H2O, the coefficients are mole ratios: 2 mol of H2 react with 1 mol of O2 to give 2 mol of water. Masses do not follow those ratios; moles do.
Mistakes that come up
- Using mass ratios in place of mole ratios. Convert to moles before you use the coefficients.
- Mixing up moles of molecules and moles of atoms. One mole of H2O contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms.
- Thinking a mole of every substance has the same mass. It has the same number of particles; the mass depends on the particle.
- Rounding Avogadro's number too early in a multistep problem and losing significant figures.
Picturing the count
Ask Isotoon "What is a mole and why do chemists use it?" and you get a short animated video that scales up from a handful of atoms to a sample you could weigh. To go a level down, see what an atom is and what an isotope is, which explains why average atomic masses are not whole numbers. The chemistry page lists more.
Questions
- Why is Avogadro's number exactly 6.02214076 × 10^23?
- Since the 2019 redefinition of SI units, the mole is defined by fixing Avogadro's constant at that exact value. Before then, the mole was defined as the number of atoms in 12 grams of carbon-12.
- What is the difference between molar mass and molecular mass?
- Molecular mass is the mass of one molecule, in atomic mass units. Molar mass is the mass of one mole of molecules, in grams per mole. The two have the same number.