Reacting Masses

Reacting masses tell us how much of each substance reacts or is produced in a chemical reaction. Think of a balanced equation like a recipe: the numbers in front of formulas are like cups or spoons. They compare amounts in moles, not in grams.

Key ideas

• Balanced equations show mole ratios (the “recipe”).
• Relative atomic mass, Ar, and relative formula mass, Mr, give the mass of 1 mole in grams (molar mass, M).
• A mole is a counting unit. Avogadro constant: $6.02 \times 10^{23}$ particles in 1 mole.
• Use $n = \dfrac{m}{M}$ to link mass (m), molar mass (M), and amount (n, in moles).

Method (recipe for reacting masses)

1. Balance the equation.
2. Molar masses: find M from Ar values.
3. Moles of the known substance: $n = \dfrac{m}{M}$.
4. Ratio: use coefficients to find moles of the other substance(s).
5. Mass: convert moles back to grams: $m = n \times M$.

Common mistakes

• Using coefficients as grams. They are mole ratios.
• Forgetting to balance before calculating.
• Using atomic masses for molecules like O₂ (use 32, not 16).