Chapter-3-Stoichiometry-Calculations-with-Chemical-Formulas-and-Equations-Common-Misconceptions

1. Chapter 3. Stoichiometry: Calculations with Chemical Formulas and Equations Common Misconceptions • Subscripts tell the number of atoms of each element in a molecule. Coefficients tell the number of molecules. Both of them are needed to perform stoichiometry calculations. However, once you are certain you have the proper formulas (that is to say, the correct subscripts), to balance a chemical equation you are only allowed to manipulate the coefficients. • Simply put, a mole is a collection of items just like a dozen. A dozen is a set containing 12 items; a mole is a set containing . • Just as a dozen cars does not weigh the same as a dozen bicycles, a mole of substance X does not weigh the same as a mole of substance Y (unless by coincidence). • Keep in mind that the number of moles (or the masses) that you calculate using stoichiometry, is different from the masses (or numbers of moles) we end up obtaining/manipulating in the lab. (Why?) • When calculating empirical formulas, bear in mind that the subscripts thus obtained are not exact whole numbers because of experimental or round-off errors. Every time we use experimental data to perform whatsoever calculations, there will be an error involved (therefore, sig. figs. are a must). • Although there are different ways to find it, remember that the limiting reactant (L.R.) is the one that yields the least amount of product(s); once it runs out, the reaction stops. Therefore, the first thing to do is to find the L.R. in a stoichiometry problem. • If you calculate a percent yield in excess of 100%, is that better than calculating a yield smaller than 100%? (Yes or no, and why?) Reyes Chem 1A