edlted by
RONALDDELORENZO
Middle Georgia College Cochran. GA 31014
Mole Fraction Analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014 Many students experience difficulty with concentration units. Much of the mathematics encountered in concentration unit problems is relatively simple when isolated from the chemistry. Students become confused when these mathematical concepts are presented as an integral part of the chemical problem, because it is usually the chemistry itself with which they are uneasy. The analogy discussed in this paper accomplishes two goals. First, i t convinces the student that the mathematics is not difficult, and, second, i t gives the student a better understanding of the chemical problem. Many textbooks and teachers present the concept of mole fraction merely as a formula, i.e.,
where X,,I,,, is the mole fraction of solute, n,,l,, w d n,,l,,., are the number of moles of solute and solvent, respectively. T o many students, thissays very little. Studenw view this as anothe;formula to he memorized and used without logic or comprehension. hisa author has had good results by presentingrlasses with analogous problems hefore presenting formal chemical deli-
nitions and Droblems. The followine - is an illustration of this approach. Class Question: If a class is made UD of three -girls and three boys, whit fraction of the class is female? Many students will "intuitively" know that the class is 50% or one half female (i.e., the female fraction of the class is '12 or 0.5). The next step is to show the class how the "%" answer is obtained, i.e., number of girls -- -3 -- -1 -- 0,6 female fraction = total number of students 6 2 female fraction =
number of girls number of girls + number of boys
-
3 -3-1 3+3 6 2
Most students are then able to handle an analogous chemical problem such as the following. Class Question: If a solution is made up of three moles of water and three moles of alcohol, what fraction of the solution is alcohol? Taken from DeLorenzo, Ronald, "Chemistry:Problem Solving and Comprehensian Enhancements Through Applications, Anecdotes, and Analogies," D.C.Heath and Company, Lexington, Massachusetts, 1981.
Volume 57, Number 10, October 1980 1 733
