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:
Clues that it IS NOT a limiting reagent problem:
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.
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:
- The question asks you to find the limiting reagent (reactant).
- TWO OR MORE REACTANT amounts are given.
Clues that it IS NOT a limiting reagent problem:
- The question only gives you the amount of one reactant or it does not give you the amount of any of the reactants.
- The question mentions that all but one of the reactants are present in excess.
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.
Last modified: Thursday, August 5, 2010, 9:56 AM