Lecture Presentation Chapter 3 Stoichiometry Calculations with Chemical Formulas and Equations John D Bookstaver St Charles Community College Cottleville MO 2012 Pearson Education Inc Law of Conservation of Mass We may lay it down as an incontestable axiom that in all the operations of art and nature nothing is created an equal amount of matter exists both before and after the experiment Upon this principle the whole art of performing chemical experiments depends Antoine Lavoisier 1789 2012 Pearson Education Inc Stoichiometry Chemical Equations Chemical equations are concise representations of chemical reactions 2012 Pearson Education Inc Stoichiometry Anatomy of a Chemical Equation CH4 g 2O2 g CO2 g 2H2O g 2012 Pearson Education Inc Stoichiometry Anatomy of a Chemical Equation CH4 g 2O2 g CO2 g 2H2O g Reactants appear on the left side of the equation 2012 Pearson Education Inc Stoichiometry Anatomy of a Chemical Equation CH4 g 2O2 g CO2 g 2H2O g Products appear on the right side of the equation 2012 Pearson Education Inc Stoichiometry Anatomy of a Chemical Equation CH4 g 2O2 g CO2 g 2H2O g The states of the reactants and products are written in parentheses to the right of each compound 2012 Pearson Education Inc Stoichiometry Anatomy of a Chemical Equation CH4 g 2O2 g CO2 g 2H2O g Coefficients are inserted to balance the equation 2012 Pearson Education Inc Stoichiometry Subscripts and Coefficients Give Different Information Subscripts tell the number of atoms of each element in a molecule Coefficients tell the number of molecules Stoichiometry 2012 Pearson Education Inc Reaction Types 2012 Pearson Education Inc Stoichiometry Combination Reactions In combination reactions two or more substances react to form one product Examples 2Mg s O2 g 2MgO s N2 g 3H2 g 2NH3 g C3H6 g Br2 l C3H6Br2 l 2012 Pearson Education Inc Stoichiometry Decomposition Reactions In a decomposition reaction one substance breaks down into two or more substances Examples CaCO3 s CaO s CO2 g 2KClO3 s 2KCl s O2 g 2NaN3 s 2Na s 3N2 g 2012 Pearson Education Inc Stoichiometry Combustion Reactions Combustion reactions are generally rapid reactions that produce a flame Combustion reactions most often involve hydrocarbons reacting with oxygen in the air Examples CH4 g 2O2 g CO2 g 2H2O g C3H8 g 5O2 g 3CO2 g 4H2O g Stoichiometry 2012 Pearson Education Inc Formula Weights 2012 Pearson Education Inc Stoichiometry Formula Weight FW A formula weight is the sum of the atomic weights for the atoms in a chemical formula So the formula weight of calcium chloride CaCl2 would be Ca 1 40 08 amu Cl 2 35 453 amu 110 99 amu Formula weights are generally reported for ionic compounds 2012 Pearson Education Inc Stoichiometry Molecular Weight MW A molecular weight is the sum of the atomic weights of the atoms in a molecule For the molecule ethane C2H6 the molecular weight would be C 2 12 011 amu H 6 1 00794 amu 30 070 amu 2012 Pearson Education Inc Stoichiometry Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation Element number of atoms atomic weight FW of the compound x 100 Stoichiometry 2012 Pearson Education Inc Percent Composition So the percentage of carbon in ethane is C 2 12 011 amu 30 070 amu 24 022 amu 30 070 amu 79 887 x 100 2012 Pearson Education Inc Stoichiometry Moles 2012 Pearson Education Inc Stoichiometry Avogadro s Number 6 02 x 1023 1 mole of 12C has a mass of 12 000 g 2012 Pearson Education Inc Stoichiometry Molar Mass By definition a molar mass is the mass of 1 mol of a substance i e g mol The molar mass of an element is the mass number for the element that we find on the periodic table The formula weight in amu s will be the same number as the molar mass in g mol 2012 Pearson Education Inc Stoichiometry Using Moles Moles provide a bridge from the molecular scale to the real world scale 2012 Pearson Education Inc Stoichiometry Mole Relationships One mole of atoms ions or molecules contains Avogadro s number of those particles One mole of molecules or formula units contains Avogadro s number times the number of atoms or ions of each element in the compound Stoichiometry 2012 Pearson Education Inc Finding Empirical Formulas 2012 Pearson Education Inc Stoichiometry Calculating Empirical Formulas One can calculate the empirical formula from the percent composition 2012 Pearson Education Inc Stoichiometry Calculating Empirical Formulas The compound para aminobenzoic acid you may have seen it listed as PABA on your bottle of sunscreen is composed of carbon 61 31 hydrogen 5 14 nitrogen 10 21 and oxygen 23 33 Find the empirical formula of PABA 2012 Pearson Education Inc Stoichiometry Calculating Empirical Formulas Assuming 100 00 g of para aminobenzoic acid C H N O 61 31 g x 5 14 g x 10 21 g x 23 33 g x 1 mol 12 01 g 1 mol 1 01 g 1 mol 14 01 g 1 mol 16 00 g 5 105 mol C 5 09 mol H 0 7288 mol N 1 456 mol O 2012 Pearson Education Inc Stoichiometry Calculating Empirical Formulas Calculate the mole ratio by dividing by the smallest number of moles 7 005 6 984 5 105 mol 0 7288 mol 5 09 mol 0 7288 mol 0 7288 mol 0 7288 mol 1 000 1 458 mol 0 7288 mol 2012 Pearson Education Inc 2 001 C 7 H 7 N O 2 Stoichiometry Calculating Empirical Formulas These are the subscripts for the empirical formula C7H7NO2 2012 Pearson Education Inc Stoichiometry Combustion Analysis Compounds containing C H and O are routinely analyzed through combustion in a chamber like the one shown in Figure 3 14 C is determined from the mass of CO2 produced H is determined from the mass of H2O produced O is determined by difference after the C and H have been determined 2012 Pearson Education Inc Stoichiometry Stoichiometric Calculations The coefficients in the balanced equation give the ratio of moles of reactants and products 2012 Pearson Education Inc Stoichiometry Stoichiometric Calculations Starting with the mass of Substance A you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed if it s a product or used if it s a reactant 2012 Pearson Education Inc Stoichiometry Stoichiometric Calculations C6H12O6 6 O2 6 CO2 6 H2O Starting with 1 00 g of C6H12O6 we calculate the moles of C6H12O6 use the coefficients to find the moles of H2O and then turn the moles of water to grams Stoichiometry 2012 Pearson Education Inc Limiting Reactants 2012 Pearson Education Inc Stoichiometry Limiting Reactants The limiting reactant is the reactant