Reacting masses and limiting reactants · Year 12
A balanced equation tells you the ratio in which substances react, in moles. To find a reacting mass, turn the mass you know into moles, use the ratio from the equation, then turn the moles back into a mass. If one reactant runs out first it is the limiting reactant: it decides how much product forms, and the other reactant is in excess.
Key facts
- Mass is conserved: the total mass of the products equals the total mass of the reactants.
- The big numbers in a balanced equation give the mole ratio: 2Mg + O2 → 2MgO means 2 mol of Mg react with 1 mol of O2.
- Three steps: mass → moles (÷ Mᵣ), use the ratio, then moles → mass (× Mᵣ).
- The limiting reactant is the one that is used up first; the other is in excess.
- The amount of product depends on the amount of the limiting reactant.
- If a gas escapes from an open container the mass seems to fall; if a gas from the air joins on, it seems to rise.