Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 10: 11 February

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Reaction Stoichiometry, cont.

Limiting Problems

Asking question of what is the maximum amount of something which can be produced from a given mixture of stuff. This is a fairly straight-forward sort of problem in the day-to-day world, but seems to cause a great deal of difficulty for lots of folks in chemistry. Let's start by looking at a non-chemical problem:

Consider you have to make a bunch of sandwichs for a party. The equation for the sandwichs (in slices) is:

2 Bread + 1 Cheese + 1 Meat right arrow 1 Sandwich

You have a 32 oz loaf of bread, 22 oz of sliced cheese and 32 oz of sliced meat. If bread slices weigh 1/2 oz, cheese slices 3/4 oz and meat slices 1 oz, how many sandwichs can you make?

Look at how many sandwichs can be made from each ingredient:

Cheese limits and we can make 29 sandwichs.

Fe3O4 + 4 C right arrow 3 Fe + 4 CO

What is the maximum mass of Fe which could be made from 115.0 g Fe3O4 and 24.00 g C?

The trick here is to find the maximum amount of iron which could be made from each reactant.

The lesser amount will then be the max possible:

C: (3 mol Fe/ 4 mol C)(24.00 g C/ 12.01 g C/mol C) = 1.499 mole

Fe3O4: (3 mol Fe/ mol Fe3O4)(115.0 g Fe3O4/231.6 g Fe3O4/mol Fe3O4) = 1.490 mole

therefore Fe3O4 limits, can only make 1.490 moles Fe. 

Grams = (1.490 moles Fe)(55.85 g Fe/mol Fe) = 83.217 g Fe = 83.22 g Fe

Zn + 2H+ right arrow Zn2+ + H2(g)

What is the maximum amount (moles) of hydrogen gas which may be produced by reacting 0.50 g of Zinc with 0.800 mole hydrogen ion? Show work!

For Zn limiting: (1 mole H2/1 mole Zn)(0.50 g Zn) / (65.39 g Zn/mol) = 7.646 x 10-3 mol H2

For H+ limiting: (1 mole H2/2 mole H+)(0.800 mole H+) = 0.400 mol H2

Much less with Zn therefore, 7.6 x 10-3 mol H2

Percent Yield

Another frequent question arising in chemical processes is the percent yield. This deals with the question of how effective was a given process in producing a product. Its an important consideration because chemical reactions rarely go completely to products. The maximum possible yield for a reaction is known as the Theoretical Yield.

Zn + 2H+ right arrow Zn2+ + H2(g)

(6.75 x 10-3 mol)(100 %) / (7.6 x 10-3 mol) = 88.8% = 89%

Chapter 4: Aqueous Solutions

Definitions:

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© R A Paselk

Last modified 11 February 2015