Transcript You use more

Chapter 9
Mass – Mass Calculations
2Al + 6HCl → 2AlCl3 + 3H2
• How many mol Al will react with 6.37 mol HCl?
2.12 mol Al
C6H12O6 + 6O2 → 6CO2 + 6H2O
• How many grams of O2 are needed to react with
12.5 g of C6H12O6?
• How is this problem different than the previous
problem?
• This is an example of a mass – mass calculation.
General Plan for
Mass – Mass Calculations
Mass of
Different
Substance
Mass of
substance
Moles of
substance
Stoichiometric
factor
Moles of
Different
Substance
C6H12O6 + 6O2 → 6CO2 + 6H2O
• How many grams of O2 are needed to react with
12.5 g of C6H12O6?
13.3g O2
2KClO3 → 2KCl + 3O2
• If 80.5 g O2 are produced how many grams
of KCl will be produced?
125g KCl
2Al + 6HCl → 2AlCl3 + 3H2
• 9.23g of Al will produce how many
grams of H2?
1.03g H2
Percent Yield
Theoretical Yield vs. Actual Yield
• Theoretical yield is the amount of product we
should produce from the complete conversion
of a given amount of reactant to product
– must be calculated or given.
• Actual yield is the amount of product that is
actually produced during a reaction.
– it is always less than theoretical yield because of
incomplete reactions, impure reactants, side
reactions, etc
– must be given within the problem or measured
during an experiment.
Making Popcorn
• If we started with 80
popcorn kernels and found
only 72 of them popped.
• What is the percent yield of
popcorn?
Percent Yield
actual yield
% yield =
x 100%
theoretical yield
A reaction was supposed to make 2.50 grams of Zn. You
actually made 2.32 grams. What is the percent yield?
92.8%
2Al + 3CuCl2 → 3Cu + 2AlCl3
• A chemistry student is trying to prepare copper metal
by the reaction of aluminum with copper (II) chloride.
→
2Al + 3CuCl2 → 3Cu + 2AlCl3
• If the student is able to prepare 3.67g of copper metal
by the reaction of 1.27g of aluminum with excess
copper (II) chloride.
• How do I know that the aluminum gets used up and
the copper (II) chloride does not?
This is the
concept of
the “limiting
reactant”
2Al + 3CuCl2 → 3Cu + 2AlCl3
• If the student is able to prepare 3.67g of copper metal
by the reaction of 1.27g of aluminum with excess
copper (II) chloride. What is the percent yield?
Chromium (III) hydroxide will dissolve in concentrated
sodium hydroxide according to the following equation:
NaOH + Cr(OH)3  NaCr(OH)4
If you begin with 66.0g of Cr(OH)3 and obtain 38.4g of
NaCr(OH)4, what is the percentage yield?
41.9%
Homework
• “Mass-Mass Problems” Worksheet
Two other types of percent yield problems
•
In both of these types of problems
you will be given the percent yield.
1. Given the percent yield determine the
amount of a reactant you need.
– You use more.
2. Given the percent yield determine the
amount of a product you can produce.
– You make less.
You use more.
• A bakery produces cookies at
a percent yield of 92%. A
customer places an order for
2150 cookies. The bakers will
need enough ingredients for
how many cookies.
2150 = 2337 cookies
0.92
Divide by percent yield. “You use more”
You make less
• A bakery produces
cookies at a percent
yield of 92%. The
bakers add enough
ingredients to make
1525 cookies. How
many cookies do they
produce?
1525 (0.92) = 1403 cookies
Multiply by the percent yield. “You make Less”
A chemical engineer needs to produce 212 g of zinc
oxide by the process below:
2ZnS + 3O2  2ZnO + 2SO2
By doing trials runs he expects the process to run at a
78% yield. How much zinc sulfide should he use to
prepare the 212 g of zinc oxide?
326g ZnS
Ammonia is synthesized from hydrogen and nitrogen
according to the following equation:
N2 + 3H2  2NH3
If an excess of nitrogen is reacted with 3.41g of hydrogen,
how many grams of ammonia can be produced assuming
the reaction has a percentage yield of 48.8%?
9.42g NH3
Homework
• Lab Summary: Percent Yield Lab
• Percentage Yield Worksheet (front and
back).