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Lecture 8

CHEM 1201 Lecture 8: Limiting ReagentsPremium

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CHEM 1201

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Chem 1201 - Lecture 8 - Limiting Reagents
Limiting Reagents
The amounts of two known reactants are given, one will be used up. The other reactant
is in excess…but which one is it?
The limiting reagent is completely consumed in the reaction.
The limiting reagent limit’s the amount of product obtained.
Therefore, at the end of reaction, those reactants present in excess will still be in the
reaction mixture.
General Scheme for a Typical Limiting
GOAL (#1): Identify which reagent is the “limiting reagent”
Reagent amounts are given in gram amounts (haven’t covered molarity yet); so
these values must be converted to moles via molar mass bearing in mind the
stoichiometry of the reaction (MOST IMPORTANT!!).
The reagent with the smallest mole value, from using stoichiometry, is the limiting
Multiply the moles of the limiting reagent times the molar mass of the product
(and preserving stoichiometry) gives us the theoretical yield in grams. (i.e. moles
x molar mass = grams). This number is the theoretical yield assuming all the
limiting reagent is converted to product barring any form of experimental error.
GOAL (#2): Determine percent yield
If given an actual (or TRUE experimental value) yield one can determine a
percent yield.
Note: A chemical reaction is no guarantee that all the reagents will be consumed
to produce 100% yield due solubility and various other affects.
First let’s simplify the concept with something more appetizing before we jump into the
numbers and letters. Suppose you have a job making sundaes and each sundae
requires two scoops of ice cream, one cherry and 50 ml of chocolate syrup:
2 scoops + 1 cherry + 50 ml syrup 1 sundae
A mob of 25 first graders enters; can you feed them all? You have 50 scoops of ice
cream, 30 cherries, and 1 liter of syrup; a quick calculation shows that:
Ice cream: No. of sundaes = 50 scoops (1sundae/2 scoops) = 25 sundaes
Cherries: No. of sundaes = 30 cherries (1sundae/1 cherry) = 30 sundaes
Syrup: No. of sundaes = 1000ml syrup (1sundae/50 ml syrup) = 20 sundaes
The syrup is the limiting “reagent” here because it limits the total amount product
(sundaes) that can be produced. Of the three ingredients, the syrup allowed the
fewest sundaes (20); the rest of the ingredients, ice cream and cherries, are left
over “unreacted” when the syrup is gone, so they are present in excess.
Basic Example (Given mol’s, short cut a step, but I am emphasizing the importance of
watching stoichiometry).
2H2(g) + O2(g) 2H2O(l)
For every mol of O2 we need 2mol of H2 to make 2 mol of H2O. Based on
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