In the Laboratory www.JCE.DivCHED.org • Vol. 82 No. 8 August 2005 • Journal of Chemical Education 1219 It is a customary practice in chemical education to al- low introductory laboratory students to determine graphi- cal relationships between substances undergoing chemical change. Typical plots that illustrate the relationship between reactants and products often involve the variables of mass or moles (1, 2). This article is intended for instructors who would like to increase student involvement with graph con- struction specifically in the context of introductory labora- tory activities that involve mass relationships between reacting substances and products. I am specifically think- ing about the incorporation of graph construction in ex- periments where introductory students perform a synthesis for the first time such as the traditional syntheses of a me- tallic sulfide or a metallic oxide from their elements or a popular precipitation reaction such as the reaction between iron and copper sulfate (3). I am advocating additional graphing not only because of the benefit of providing learn- ers with additional opportunities to use higher-order cog- nitive processes but because certain mass plots represent fundamental chemical knowledge that students must know such as the law of the conservation of mass, the law of con- stant composition, limiting and excess reactants, as well as empirical formula. To incorporate graphing exercises involving mass rela- tionships into lab activities, three simple actions are neces- sary: (i) The quantities of reactants that have undergone trans- formation and the quantities of products that have been made as a result of the chemical reaction must be directly measured or deduced (ii) The mass of reactants in the experiment must be var- ied (iii) To save time, students should work in groups and pool data In regard to these conditions, I have collected and de- scribed a variety of mass plots that have been developed and used in our general chemistry program. After each different graph, I have included a set of questions along with their answers that educators might find useful to prompt students’ understanding of the graphical relationship between specific variables. While each plot is not completely novel, taken to- gether the collection is a contribution to the graphing litera- ture in chemical education. The mass plots that will be presented are based on the synthesis of zinc iodide from its elements and are taken from our instructor’s manual (4). Since these plots are not necessarily dependent upon these chemi- cal substances, educators might consider adapting the plots to the reactions they presently use to illustrate some of the above chemical concepts. The Synthesis of Zinc Iodide To show how I varied the mass of reactants to promote graph construction, I have simply required students to form groups of four, do as many trials as they can of their assigned synthesis and then pool their data. Table 1 is given out at the beginning of the laboratory activity. The procedure for the reaction has been described in a previous issue of this Journal (5). The chemical equation for this reaction is: Zn(s) + I 2 (s) + H 2 O(l) → ZnI 2 (s) + H 2 O(g) The manner in which this reaction is used in the lab calls for iodine to be the limiting reactant; any quantities of iodine larger than the ones listed here will demand more water to be used, which will prolong the isolation of the product. Hazards When properly handled, zinc and solid iodine are non- toxic or only pose slight hazards. Iodine gas is toxic, but is not produced in this activity. Zinc, iodine, and zinc iodide can be disposed of by changing the solids to ions and then diluting them in a drain attached to a sewer system at a rate that is in compliance with local authorities. Recycling of the excess zinc after the synthesis also can be done if desired. Mass Relationships in a Chemical Reaction: Incorporating Additional Graphing Exercises into the Introductory Chemistry Laboratory Stephen DeMeo Department of Curriculum and Instruction, Department of Chemistry, Hunter College of the City University of New York, New York, NY 10021; sdemeo@hunter.cuny.edu s i s e h t n y S r e p e n i d o I d n a c n i Z f o s e s s a M . 1 e l b a T d e n g i s s A s i s e h t n y S r e b m u N s s a M l a i t i n I g / c n i Z f o s s a M l a i t i n I g / e n i d o I f o f o e m u l o V L m / r e t a W 1 0 . 2 0 . 3 0 1 2 0 . 1 0 . 2 5 3 0 . 2 0 . 1 5 4 0 . 3 0 . 2 5