Transcript Stoichiometry - Midland Park School District

Ch. 11: Stoichiometry
Sec. 11.4: Percent Yield
Objectives
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
Calculate the theoretical yield of a
chemical reaction from data.
Determine the percent yield for a chemical
reaction.
Percent Yield
Suppose you were
practicing jump shots. In
your best practice, you shot
75 balls.
THEORETICALLY, you
should have been
successful 75 times. But, in
ACTUALITY, you shot 49
out of 75. To determine
your efficiency, you
calculate a percentage:
actual shots x 100
theoretical shots
= 65%
Percent Yield
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Similar calculations are made to determine
the success of chemical reactions. Most
reactions never succeed in producing the
predicted (theoretical) amount of product.
Why?



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Not every reaction goes to completion.
If liquids are produced, they may adhere to glassware.
If gases are produced, they may evaporate off.
Solids can be lost on transfer from one container to
another.
Sometimes side reactions occur, reducing the yield of
the intended product.
Calculating Percent Yield
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We have learned how to calculate the amount of
product formed from a given amount of reactant
using stoichiometry. This value is actually the
theoretical yield of a reaction. It’s how much
product forms when conditions are ideal.
Theoretical yield is the maximum amount of
product that can be produced from a given
amount of reactant.
Calculating Percent Yield
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A chemist determines the actual amount
of product formed through careful
experimentation. This is his LAB DATA.
Actual yield is the amount of product
actually produced when the chemical
reaction is carried out in an experiment.
Calculating Percent Yield
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Percent yield of product is the ratio of the actual
yield to the theoretical yield expressed as a
percent:

Remember: Actual yield comes from
experimental data; theoretical yield comes from
stoichiometric calculations.
Sample Problem

When potassium chromate (K2CrO4) is
added to a solution containing 0.500 g
silver nitrate (AgNO3), solid silver
chromate (Ag2CrO4) is formed.


Determine the theoretical yield of silver
chromate precipitate.
If 0.455 g of silver chromate is obtained,
calculate the percent yield.
Sample Problem (cont.)
1.Write a balanced equation and identify
known & unknown:
2AgNO3 + K2CrO4 Ag2CrO4 + 2KNO3
0.500 g
?g
2.Covert grams of known to moles.
0.500 g AgNO3 x 1 mole = .00294 mol
169.9 g
AgNO3
Sample Problem (cont.)
3.Use a mole ratio to get moles of unknown;
0.00294 mol AgNO3 x 1 mol Ag2CrO4 =
2 mol AgNO3
0.00147 mol Ag2CrO4
4.Calculate the mass of the product from
moles:
0.00147 mol Ag2CrO4 x 331.8 g = 0.488 g
1 mol
Ag2CrO4
Sample Problem (cont.)

This is the theoretical yield: 0.488 g Ag2CrO4
To get the percent yield, divide the actual
yield (given in the problem as 0.455 g) by
the theoretical yield & multiply by 100:
0.455g x 100 = 93.2% yield of Ag2CrO4
0.488 g
NOTE: Percent yield will NEVER be greater than
100%.
Practice Problems


If 14.0 g Al(OH)3 is present in an antacid tablet,
determine the theoretical yield of AlCl3 produced
when the tablet reacts with an excess of
stomach acid. If the actual yield is 22.0 g, what
is the percent yield?
Al(OH)3 + 3HCl  AlCl3 + 3H2O
Zinc reacts with iodine in a synthesis reaction.
Write a balanced chemical equation for the
reaction. Determine the theoretical yield if a
125 g of zinc were used. Determine the percent
yield if 116 g of product was recovered.