Harold Dorf
Midwood High School Brooklyn, New York
The "Reaction Equivalent" in Stoichiometric Problems
Stoidnionwtric pt.oblcnis bused on vhcnir:J eql~;ttions,p:~rticul:~rly prol)lr~nsin\x~lvinglimitin# reagents and a variety of units, give trouble to many students in general and analytical chemistry courses. Anyone who has graded such problems on student examinations often wonders if the students really understand the principles and mathematical manipulations involved in stoiehiometric calculations. The method here described has been successfully used for several years in Advanced Placement Chemistry classes a t Midwood High School. This method is based on a uniform logical procedure and a single simplified format, for solving all stoichiomet,ric problems based on chemical equations. I t has the advant,age of "plng-in" convenience but enhances student understanding of the basic principles of stoichiorn~tricequivalence.
t.he coefficients in thc balanced equation. Since each R E is both chemically and mat,hematically equivalent to every other RE: in the equation, all multiples or submnlt,iples of these RE'S can he mat,hematically compared or equated directly without further manipulation. The value of each RE, when expressed in the units required in the problem, gives the quantity of reagent orproduct,in the desired units, that is stoichiometrically equivalent to every other R E in the equation. When the quantity of each reactant available is expressed in RE's, the magnitudes of these RE's are directly comparable and the limiting reagent can be determined by inspect,ion. The number of RE'S of the limiting reagent, t,hen det,ermines the maximum amount of each reagent consumed or ~ r o d u cformed. t Conversion to the desirrd nnits becomes a simple one-step iiffair as shown in the solution to t.he bypical problem.
Typical Ptohlent. If 3.00 g .\I are treated with 25.0 ml 0.300 N HBO, (0.150 M ) , x h s t volume of Hr gas aill be collected over water s t 20°C, 745 n m H g atmospheric pressure? How much Ah(S0,b. 9H.O can be obtained hy ~vapurating t,he solution remaining after the reaction? Which reagent vas present in excess? How much excess?
The Method 1 . Write a halxnccd wpatian lor tlw reaction. E n c l o ~w~r h reactant and product in hrackrts. Each of the brark
