5.2 - The Mole

The mole and the mass of one mole

• Atoms are far too small to count individually, so chemists count by weighing.
• The counting unit is the mole (symbol mol) - a fixed number of particles, just as a dozen is a fixed number of items.
• The particles can be atoms, molecules, ions, electrons or formula units - whichever the substance is made from.
• The quantity measured is the amount of substance, measured in moles.

Working out the mass of one mole

The mass of one mole of a substance is its relative formula mass (or relative atomic mass) in grams.

12 g of carbon, 28 g of nitrogen gas and 44 g of carbon dioxide each hold one mole of particles, even though their masses differ.

Relative formula mass vs molar mass

• Relative formula mass is a comparison, so it has no units (for CO₂, it is 44).
• The mass of one mole is measured in grams - 44 g for CO₂. The molar mass is the mass per mole, measured in g/mol - 44 g/mol for CO₂.
• Its value is always numerically identical to the relative formula mass.

Formula for amount of substance

Where:

• Amount of substance is in moles (mol)
• Mass is in grams (g)
• $M_r$ (or $A_r$) = Relative formula mass (or relative atomic mass)

Convert masses in kilograms or tonnes to grams before calculating (1 kg = 1,000 g; 1 tonne = 1,000,000 g).

Worked example - Amount in a given mass

A sample of sodium chloride, NaCl, has a mass of 25 g. The relative atomic masses are Na 23 and Cl 35.5. What amount of sodium chloride, in moles, is this?