the stoichiometrically-calculated theoretical yield. Percent yield, then, is expressed in the following equation: If 170.0 g of lead(II) oxide is obtained, then the percent yield would be calculated as follows: Example Consider the following reaction, in which iron(III) chloride reacts with hydrogen sulfide to produce iron(III) sulfide and hydrogen chloride: The stoichiometric masses for this reaction are: 324.41 g FeCl3, 102.25 g H2S, 207.89 g Fe2S3, 218.77 g HCl Suppose 90.0 g of FeCl3 reacts with 52.0 g of H2S. To find the limiting reagent and the mass of HCl produced by the reaction, we change the above amounts by a factor of 90/324.41 and obtain the following amounts: 90.00 g FeCl3, 28.37 g H2S, 57.67 g Fe2S3, 60.69 g HCl The limiting reactant (or reagent) is FeCl3, since all 90.00 g of it is used up while only 28.37 g H2S are consumed. Thus, 52.0 − 28.4 = 23.6 g H2S left in excess. The mass of HCl produced is 60.7 g. Note: By looking at the stoichiometry of the reaction, one might have guessed FeCl3 being the limiting reactant; three times more FeCl3 is used compared to H2S (324 g vs 102 g). Different stoichiometries in competing reactions Often, more than one reaction is possible given the same starting materials. The reactions may differ in their stoichiometry. For example, the methylation of benzene (C6H6), through a Friedel–Crafts reaction using AlCl3 as a catalyst, may produce singly methylated (C6H5CH3), doubly methylated (C6H4(CH3)2), or still more highly methylated (C6H6−n(CH3)n) products, as shown in the following example, C6H6 + CH3Cl → C6H5CH3 + HCl C6H6 + 2 CH3Cl → C6H4(CH3)2 + 2 HCl C6H6 + n CH3Cl → C6H6−n(CH3)n + n HCl In this example, which reaction takes place is controlled in part by the relative concentrations of the reactants. Stoichiometric coefficient and stoichiometric number In lay terms, the stoichiometric coefficient of any given component is the number of molecules and/or formula units that participate in the reaction as written. A related concept is the stoichiometric number (using IUPAC nomenclature), wherein the stoichiometric coefficient is multiplied by +1 for all products and by −1 for all reactants. For example, in the reaction , the stoichiometric number of CH4 is −1, the stoichiometric number of O2 is −2, for it would be +1 and for H2O it is +2. In more technically precise terms, the stoichiometric number in a chemical reaction system