Limiting Reagents
What’s gonna run out first?
In almost any chemical reaction, one reagent
(reactant) is completely used up before another.
When all of one reactant is used up, the reactions
STOPS. Therefore, whatever this limits the extent of
the reaction making it the LIMITING REAGENT.
Real Life
Your job is to make as many cheese burgers as you
possibly can.
You have 4 buns, 6 cheese slices, 3 patties, and 7 tomato
slices (2 per burger). How many cheese burgers can
your make?
You can identify these questions because they have TWO GIVENS.
FIRST: FIND THE LIMITING REAGENT!
> Convert BOTH givens into moles of wanted
> Whichever one produces less moles of your wanted substance
is the limiting reactant.
SECOND: USE THE LIMITING REAGENT TO DO ALL
CALCULATIONS
> Used the limiting reagent as your given and solve just like any
other stoichiometry problem.
In the reaction:
2AgNO3 + CaCl2 →2AgCl + Ca(NO3)2
how many grams of AgCl will precipitate (form) when 20.0g€AgNO3
is reacted with 15 grams of€CaCl2?
FIND THE LIMITING REAGENT
Given #1______________
Given #2______________
Wanted_______________
AgNO3
143.32g
(20.0g of AgNO3) 1 mol 2 mol of AgCl
169.88g 2 mol of AgNO3 1 mol
= 16.87 g of AgCl
CaCl2
(15.0g of CaCl2)
1 mol 2 mol of AgCl
110.98g 1 mol of CaCl2
143.32g
1 mol
= 38.74 g of AgCl
Since AgNO3 produced less AgCl, AgNO3 is the
limiting reagent, and there will be excess CaCl2 that
does not react with anything.
79.1g of Zinc react 2.25moles of HCl. Identify the
limiting reactant. How many litres of hydrogen are
formed at STP?