When reagents are not mixed in exact stoichiometric proportions, one will be used up first — this is the limiting reagent. The other reagent is in excess.
Key Concept
The limiting reagent determines the maximum amount of product that can be formed. Once it is consumed, the reaction stops — regardless of how much excess reagent remains.
How to Identify the Limiting Reagent
Limiting Reagent Method
Think About It
In the example above, how much excess sulphur (in grams) remains after the reaction?
Moles S used = 0.0895 mol (same as Fe, 1:1 ratio). Moles S remaining = 0.156 − 0.0895 = 0.067 mol. Mass remaining = 0.067 × 32.07 = 2.1 g.