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Stoichiometry is traditionally a topic that is covered in each of the different levels at Argo Community High School (Argo). While the Honors Chemistry and Advanced Placement (AP) Chemistry courses stress a mathematical approach to mass and mole relationships, the Regular Chemistry course typically receives a lighter, scaled back approach due to a lower level of math skill and achievement. In Birdâs âStoichiometric Calculations Using Equivalent Molar Expressionsâ, the argument is made for a different mathematical approach using equivalent molar expressions instead of the standard unit or dimensional analysis method (2006). While this method does have some unique benefits for all students it would be difficult to implement completely at all levels of chemistry at Argo. One of the best aspects of using âequivalent molar expressionsâ is it shows the importance of the relationship of a balanced equation to the reacting moles (Bird, 2016). Students at Argo currently have two paths into Chemistry; a traditional biology first sequence and a nontraditional physics first sequence. The teachers in the freshman physics first sequence stress dimensional analysis throughout the course. For Sophomore Honors and AP Chemistry students stoichiometry occurs early in the semester and is continued throughout the school year. Although students have had at least one previous year using dimensional analysis, their proficiency in using dimensional analysis determines the studentsâ success or failure with stoichiometry. Students often rely on rote methods to arrive at a final answer but cannot always explain the value they calculated. Using the molar expressions method, each equivalent mole of reactant or product is determined by using the relationship of the coefficient to the starting or ending moles in the reaction. Through this method the students can see that the moles are equivalent and interchangeable, thus making a stronger connection to a particle approach to stoichiometry. Instead of a rote method of a series of dimensional analysis problems the student can quickly and easily determine the moles or other quantity desired in a balanced equation. Another positive aspect of the molar expression method is its flexibility in use with various topics. This method, combined with an ICE/BCA table (Initial, Change, End or Before, Change, After) approach, can be used to quickly inform the student of how much excess remains, how much product is formed, and what is present in the reaction vessel at the end of a reaction (May, 2017). Students in Honors and AP Chemistry at Argo struggle with determining the amount of excess reactant that remains at the end of a reaction. Various wrong methods to determine excess remaining are applied as it is a different calculation than their rote processes previously developed. Using molar expressions, combined with an ICE/BCA table, students can quickly see the amount remaining and will more accurately understand the process of the limiting reactants and the reaction itself. The students can also see that at the end there are some reactants and products remaining. Combining molar expressions with an ICE/BCA table has an added benefit of introducing ICE tables earlier in the curriculum. Although using equivalent moles can simplify stoichiometry calculations while also clarifying many mathematical misconceptions, the worst argument is the inclusion of the actual molar expression to the calculations as it is an unnecessary addition and one that would be detrimental to understanding the stoichiometry of the reaction. When students see a myriad of equations for the numerator they either quickly look for a method to memorize all of these diverse equations or simply assume it is too complicated to understand. If instead the mole was used and equivalent moles was determined instead of equivalent molar expressions, the work would become less clouded with variables. High School students may lose vigor when confronted with such a problem. Instead the students should rely on their ability to convert into moles by using molarity, the ideal gas law, or simply grams to moles, and then use equivalent moles, the process of stoichiometry can be kept separate from the method of determining moles in various ways. Using equivalent molar expressions, or just equivalent moles as I have argued, a connection can be made to both Extreme Whoosh Bottle Trio and Big Time Ethyl Alcohol Explosion videos provided by Flinn (Bracken, Dombrink, & Long, 2017; Marek, 2017). To make the demonstrations quantitative you could simply measure the amount of water generated in each demonstration. The Extreme Whoosh Bottle Trio lends itself particularly to a quantitative treatment as three different concentrations of alcohol were used. By determining the amount of water that should be generated by the 70% isopropyl solution and then comparing it to the amount of water that is actually generated, students can quickly see the application of stoichiometry as well as the use of equivalent molar expressions. The same treatement can be applied to the pure ethanol as well as the 95% isopropyl, allowing several examples as either an introduction or a summation set of demonstrations. The general idea of the equivalent molar expressions is applicable and should be used in all classes at Argo, with some modification (Bird, 2016). While the AP and Honors Chemistry students can definitely use the molar expression within the ICE/BCA table described within this paper, I have argued that the students will find it simpler without the molar expression, instead just using moles in the expression. But it would be advantageous to use molar expressions at the end of the year as a mathematical summation of several topics including percent composition, molarity, and ideal gases. However, the Regular Chemistry students would see no benefit of including molar expressions into the ICE/BCA table. The complexity of several different possible numerators would prove to be too cumbersome. Instead, it would be more appropriate to implement equivalent moles in those classes.
