7.5 PERCENTAGE
YIELD
Percent Yield
• Not all chemical reactions will go to completion
of until all the limiting reagent is used up.
• In past calculations, we have assumed that the
reaction will continue until one of the reactants is
used up (limiting reagent).
• There are several possible reasons for not
obtaining the maximum possible amount of
product:
• The nature of the reaction
• Some reactions, by nature, are extremely difficult to perform
• Sometimes the reaction achieves an equilibrium before all the reactants are
converted into products
• The experimental procedure
• Materials are lost through transfer (ie: residue remains in the beaker, splattering
occurs during heating, spillage)
• Impurities
• Chemicals are rarely 100% pure; pure chemicals are expensive!
• Aging can change the composition of a substance (ie: powders may absorb water
from the air over time)
• Competing side reactions
• Other side reactions occur at the same time, using up your product (byproducts are
formed)
Theoretical Yield
• In an ideal chemical reaction, all of the limiting
reagent is converted into the desired product, giving
the maximum possible yield of the product.
• 100% of the limiting reagent is converted into
products
Actual Yield
• Usually, the above does not actually happen.
• The actual yield is how much product is actually
produced or collected during an experiment.
• Value between 0-100%
• You want to get as close to 100% as possible!
Calculating Percentage Yield
• When we know the actual yield, the percent yield can be calculated as follows:
• Can use either mass or mols for “yield”
Tips:
• Use stoichiometry to determine the theoretical yield of the desired product
• If you are not told what the limiting reactant is, you must first determine it!
• Actual and theoretical yield must be in the same units (both grams or both
mols)
EXAMPLE
Methanol (CH3OH) can be made in a reaction using carbon dioxide and
hydrogen. Water is the other product. During an investigation, 20.0g of
hydrogen was reacted with excess carbon dioxide to produce 102.0 g of
methanol. What is the percentage yield of this reaction?
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