Problem-~olving skills inchemistry ~~d~ -E Kenneth V. Fast ~ i r k w w dMissouri ~ i a h Schwl. Kirkwood. MO
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Teaching problem-solving skills to the h e g i n w chemstry student mav he made easier for both student and instructor when the student learns to use a simple swp-by-stepformat in which the student8 use thought processes rather than a set formula for problem solutions. The method described here is adaptable to solving problems related to gas laws, the mole concept (including the balanced eauation), molarity, heats of reactions, and other chemistry concepts encountered by students in beginning chemistry. Dimension analysis, factor analysis, and significant-figure skills are utilized in the format in a way that the student may more clearly understand the importance of these skills in prohlem solving. The format also enables the student to enter problem values in a calculator in a systematic way for numerical answers. The concrete level of learning requires the step-hy-step format and the recall and comprehension of principles and concepts necessary for solving given prohlek. The formal level of learning requires the student toanalyze the prohlem then design a ioluiion using the data stated in the given prohlem. The data may he stated graphically and thus require further analysis and evaluation before the student actually formulates a solution for the problem. The format used involves thk use of'"road maps" tu lead the student through the prohlem and a logic approach to examining what is needed to solve the prohlem. Consider the following example. What volume of dry oxygen gas measured a t STP is required to react completely with 6.06 g of hydrogen gas? What is needed in order to solve this problem?
a
The format and numerical answer are easy to read. Errors can he spotted and diagnosed readily by the instructor. I believe this method and format is a bridge between the concrete and formal levels of learning. I t is useful for the heainnna student in chemistry problem solving.
Using a Conversion Matrix to Simplify Stoichiometric Calculations From Balanced Equations Selman A. Berger City University of New York John Jay College of Criminal Justice
DepaRmem of Sciences 445 West 59 Street New York. NY 10019
simplify the The following method has been devised teaching of stoichiometric calculations using conversion factors. Beginning with a balanced equation, equate the number of moles of each species using the coefficients in the balanced equation and write the equivalent number of grams of each species (coefficient X G.M.W.) directly under its corresponding number of moles. The result is an overall conversion matrix for all the species in the reaction that are mutually equivalent as indicated by the symbol 2.
2 mol NzHh = I mol NzOl 5
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5
= 4 mol HPO =
64.12 g N2Ha= 92.02 g NzOr = 84.06 g Nz = 64.08 g H20
1) correct formulas of all reactants and products far the reac-
tion 2) knowledge that the reaction does take place and the balanced equation for the reaction 3) identification of the known value given in the problem 4) conversion of the known value to moles 5) conversion of moles of known to moles of unknown using the balanced equation far necessary information fib ., conversion ..-~ . - of - moles ~ of ~ unknown ~ ~ to the desired units asked for in the problem A road map analogy is useful: Start Finish (answer)
= 3 mol Nl
=
In order to demonstrate the use of this overall conversion matrix a sample problem is solved. Example 1 Given 20 g of NzH4, find the number of g of Nz produced. Since NzH4and Nz are the only species involved in the problem, a mini-matrix is used.
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Correct formulas & -grams of Hz -moles of Hz balanced equation (Steps 1&2) (Step 3) (Step 4)
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The balanced equation reads 2Hz + O2 = 2 H 2 0 (6.06 g) (?I) The format for the solution then is as follows
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moles of O2
12m01N Z H ~ = 3 m o l ~ z grams of 0 z
1~ i t e rofs 0% (Step 5)
B
A
(Step 6)
64.12 g N2H,
D
'
84.06 g Nz C
Since a conversion of 20 g NzH4intox gof Nz is necessary, the min-matrix can he helpful because it can serve to demonstrate the appropriate cycle for the conversion needed. In this case a three-step conversion from D A B C is necessarv. .. as follows
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Ediior's note: This month's issue includes some insights into problem solving. The three amors briefly describe methods they have found useful in relating me malbmatics of calculations involving Uw mole
concept. 396
Journal of Chemical Education
