chapter 9 mass – mass calculations. 2al + 6hcl → 2alcl 3 + 3h 2 how many mol al will react with...
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![Page 1: Chapter 9 Mass – Mass Calculations. 2Al + 6HCl → 2AlCl 3 + 3H 2 How many mol Al will react with 6.37 mol HCl? 2.12 mol Al](https://reader036.vdocuments.us/reader036/viewer/2022062304/56649e745503460f94b75300/html5/thumbnails/1.jpg)
Chapter 9
Mass – Mass Calculations
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2Al + 6HCl → 2AlCl3 + 3H2
• How many mol Al will react with 6.37 mol HCl?
2.12 mol Al
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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.
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Mass of substance
StoichiometricfactorMoles of
substanceMoles ofDifferentSubstance
Mass of DifferentSubstance
General Plan for Mass – Mass Calculations
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C6H12O6 + 6O2 → 6CO2 + 6H2O
• How many grams of O2 are needed to react with 12.5 g of C6H12O6?
13.3g O2
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2KClO3 → 2KCl + 3O2
• If 80.5 g O2 are produced how many grams of KCl will be produced?
125g KCl
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2Al + 6HCl → 2AlCl3 + 3H2
• 9.23g of Al will produce how many grams of H2?
1.03g H2
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Percent Yield
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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.
• Theoretical yield is calculated from the amount of limiting reactant.
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Making Popcorn
• If we started with 80 popcorn kernels and found only 72 of them popped.
• What is the percent yield of popcorn?
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Percent Yield
actual yield% yield = x 100%
theoretical yield
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A reaction was supposed to make 2.50 grams of Zn. You actually made 2.32 grams. What is the percent yield?
92.8%
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2Al + 3CuCl2 → 3Cu + 2AlCl3
• A chemistry student is trying to prepare copper metal by the reaction of aluminum with copper (II) chloride.
→
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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”
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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?
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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%
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Homework
• “Mass-Mass Problems” Worksheet
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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.
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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 0.92
= 2337 cookies
Divide by percent yield. “You use more”
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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”
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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
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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
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Homework
• Lab Summary: Percent Yield Lab
• Percentage Yield Worksheet (front and back).