Tips for Identifying Limiting Reactant Problems


One of the many difficulties that arises with limiting reagent problems is that students often have trouble distinguishing them from typical mol-mol or mass-mass problems. Here are some tips:


Clues that it IS a limiting reagent problem:

1. The question asks you to find the limiting reagent (reactant).
   
2. TWO OR MORE REACTANT amounts are given.

Clues that it IS NOT a limiting reagent problem:

1. The question only gives you the amount of one reactant or it does not give you the amount of any of the reactants.
   
2. The question mentions that all but one of the reactants are present in excess.

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Examples:

C + O₂ ==> CO₂

A) 20.0 mol of C is mixed with 10.0 mol of O₂. What is the limiting reagent?
This is a limiting reagent problem, because 1) it asks for the limiting reagent, and 2) the amounts of two reactants (20.0 mol C & 10.0 mol O₂) are given.


B) 20.0 g of C is mixed with 100.0 g of O₂. How many grams of CO₂ will form?
This is also a limiting reagent problem, because the amounts of two reactants (20.0 g of C & 100.0 g of O₂) are given.


C) If 90.0 mol of O₂ is reacted, how many mol of C are needed?
This is NOT a limiting reagent problem. Only the amount of one of the reactants (90.0 mol of O₂) was given.


D) If 200.0 g of O₂ is reacted with excess C, how many g of CO₂ will be produced?
This is NOT a limiting reagent, because one of the two reactants (carbon) is present in excess. This leaves only one reactant (O₂) that has a finite amount.