Limiting reactant problems follow on directly from reacting mass calculations. In these questions, you have to figure out which reactant is in excess and which will limit the amount of product formed.
What does that mean?
Let’s take a familiar reaction – magnesium ribbon dropped into hydrochloric acid to produce magnesium chloride and hydrogen.
Mg(s) + 2HCl(aq) ⇾ MgCl2(aq) + H2(g)
Our magnesium ribbon strip weighs 0.2g which is 0.0083mol. Magnesium reacts with hydrochloric acid in 1:2 ratio so if I have less than 0.016mol of acid in my reaction then the magnesium will be in excess (some will be left over at the end of the reaction) and the acid will be the limiting reagent. If I have more than 0.016mol of acid, then all the magnesium will react (it is the limiting reagent now) and the acid will be in excess.
The number of moles of the limiting reagent will determine the number moles of product formed in a reaction.
Practice questions
- How much copper can be extracted from 25g copper oxide and 3g of carbon?
2CuO(s) + C(s) ⇾ 2Cu(s) + CO2(g)
2. Calculate the mass of aluminium chloride produced from 4g of aluminium and 12g of chlorine.
2Al(s) + 3Cl2(g) ⇾ 2AlCl3(s)
