Stoichiometry Mastery Exams

The Stoichiometry Mastery Exam must be passed at the 85% level or better. It deals with equations and manipulations of chemical algebra based on the various conservation laws and definitions that introduce the course. The Exam is offered six times during the course. It must be completed successfully to obtain a passing grade in the course if a passing grade is otherwise justified by performance on exams, homeworks and quizzes. But the Mastery Exam score is not otherwise factored into the grade calculation. If the Mastery Exam requirement is not satisfied, the entire course must be repeated. (You do not receive an "incomplete" grade if all that remains is the Mastery Exam.) The types of calculations that can appear on the Exam are drawn from the following categories (which can serve as a preparation guide):

atomic weights balancing equations
molecular weights reaction yields
isotopic composition limiting reagents*
moles combustion reactions
percent composition mixtures*
empirical formulas density
molecular formulas dilution problems

 

*HINTS: From a number of years of experience with difficulties some students have in dealing with the Stoichiometry Mastery Exams, I have concluded that there is a small subset of pitfalls that can be avoided by paying attention to the following hints. Learn to recognize i.) a limiting reagent problem, and ii.) a mixture problem.

For limiting reagent problems, the appropriate tactic is practice, practice, practice. There are lots of examples in the archived exams below. Always keep in mind that balanced equations for reactions involve numbers. Read the equation. Then, if this seems to work for you, re-read it in terms of "dozens". Finally, re-read it in terms of "moles", which is just a very, very large number. Once you recognize what's going on with moles, you can deal with the algebra of the problem by converting masses of materials to moles of materials. Work with the exams without answers.

For the mixture problems, a key word in the phrasing of the question is "mixture". Look for it. An example of the major stumbling block will serve well. Consider the combustion of methane CH4 + 2O2 --> CO2 + 2H2O. Now separately, look at the combustion of ethane 2C2H6 + 7O2 --> 4CO2 + 6H2O. When dealing with "a mixture of methane and ethane", you cannot work with CH4 + 2C2H6 + 9O2 --> 5CO2 + 8H2O because that unjustifiably assumes you have half the number of methanes as ethanes in the mixture. But the relative amounts of methane and ethane are what you are trying to solve for in the problem. There are an infinite number of balanced equations that satisfy the combustion of methane and ethane; like CH4 + 8C2H6 + 30O2 --> 17CO2 + 26H2O to pick a random example. That's also balanced. To continue, suppose you had exactly 25 grams of the mixture. You would proceed by setting up the reaction of x grams of methane according to its independent balanced equation and also, 25-x grams of ethane for its balanced equation. You can subsequently generate the amount of products in terms of x and equate that to the amount of a specific problem given in the wording of the question. See the many examples in the archived exams below.

Also, in an equation such as that for oxidizing ethane 2C2H6 + 7O2 --> 4CO2 + 6H2O, the molecular weight of carbon dioxide is (12.01)+2(16.00)=44.01, not 4 times 44.01. The "four" is a coefficient, not part of the molecular weight calculation.

 

Spring 2012

Mastery Exam I
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Mastery Exam I
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Mastery Exam II
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Mastery Exam V
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Mastery Exam V
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Mastery Exam VI
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Previous Mastery Exams:

Fall 2010

Mastery Exam I
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Mastery Exam II
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Mastery Exam III
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Mastery Exam IV
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Mastery Exam IV
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Mastery Exam V
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Mastery Exam V
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Fall 2009

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Mastery Exam II
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Mastery Exam III
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Mastery Exam IV
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Fall 2008

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Mastery Exam II
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Mastery Exam III
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Mastery Exam IV
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Mastery Exam V
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Fall 2007

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Mastery Exam II
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Mastery Exam III
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Mastery Exam IV
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Mastery Exam V
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Mastery Exam VI
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Fall 2006

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Mastery Exam II
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Mastery Exam III
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Mastery Exam V
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Fall 2005

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Mastery Exam II
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Mastery Exam III
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Mastery Exam IV
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Mastery Exam V
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