Amount of substance, Avogadro's constant and molar volume

Amount of substance, Avogadro's constant and molar volume

Chemistry needs a bridge between particles too small to count individually and measurable masses or volumes. The mole is that bridge. One mole contains Avogadro's constant, approximately 6.02 × 10²³ particles. At the same stated temperature and pressure, equal volumes of gases contain equal numbers of particles; at RTP, one mole of an ideal gas is commonly treated as occupying 24 dm³. This does not mean every gas sample always has that volume: conditions must be stated. A good answer identifies whether the quantity is particles, moles, mass or volume before choosing a relationship.

Work it through

A 72 dm³ gas sample at RTP contains n = V/24 = 72/24 = 3.0 mol. If asked for particles, multiply 3.0 mol by 6.02 × 10²³ particles mol⁻¹. The moles are the central quantity; do not jump from volume directly to mass without a molar mass.

Mastery target

Move between particles, moles and gas volume at stated conditions, with each quantity and unit clearly labelled.