Relative Mass, Moles and Conservation

Relative formula mass (Mr)

Add the relative atomic masses (Ar) of all atoms in the formula.

H₂O  = (1×2) + 16 = 18
CO₂  = 12 + (16×2) = 44
CaCO₃ = 40 + 12 + (16×3) = 100

Conservation of mass

In a reaction, mass of products = mass of reactants — atoms are only rearranged.

• If mass appears to change, a gas has escaped or been absorbed from the air.

Moles

moles = mass ÷ Mr        mass = moles × Mr

• 36 g of water = 36 ÷ 18 = 2 mol.
• One mole contains the Avogadro number (6.02 × 10²³) of particles.

Concentration

concentration (g/dm³) = mass ÷ volume
concentration (mol/dm³) = moles ÷ volume(dm³)

Balancing equations

Make the number of each atom equal on both sides using big numbers in front:

2H₂ + O₂ → 2H₂O

Exam tip

If a metal gains mass when heated in air, it has reacted with oxygen — mass is still conserved overall (the oxygen came from the air).