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Chapter 3 Formulas, Equations, and Moles Chemistry, 5th Edition McMurry/Fay Atomic and Molecular Mass • The atomic masses as tabulated in the periodic table are the averages of the naturally occurring isotopes. • Mass of C = average of 12C and 13C = 0.9889 x 12 amu + 0.0111 x 13.0034 amu = 12.011 amu 2 Atomic and Molecular Mass The mass of a molecule is just the sum of the masses of the atoms making up the molecule. m(C2H4O2) = 2·mC + 4·mH + 2·mO = 2·(12.01) + 4·(1.01) + 2·(16.00) = 60.06 amu 3 Avogadro and the Mole • One mole of a substance is the gram mass value equal to the amu mass of the substance. • One mole of any substance contains 6.02 x 1023 units of that substance. • Avogadro’s Number (NA, 6.022 x 1023) is the numerical value assigned to the unit, 1 mole. 4 Avogadro and the Mole • The Mole: Allows us to make comparisons between substances that have different masses. 5 Balancing Chemical Equations • A balanced chemical equation represents the conversion of the reactants to products such that the number of atoms of each element is conserved. reactants products limestone quicklime + gas Calcium carbonate calcium oxide + carbon dioxide CaCO3(s) CaO(s) + CO2(g) 6 Balancing Chemical Equations CaCO3(s) CaO(s) + CO2(g) The letters in parentheses following each substance are called State Symbols (g) → gas (l) → liquid (s) → solid (aq) → aqueous 7 Balancing Chemical Equations A balanced equation MUST have the same number of atoms of each element on both sides of the equation. H2 + O2 → H2O Not Balanced 2H2 + O2 → 2H2O Balanced 8 Balancing Chemical Equations The numbers multiplying chemical formulas in a chemical equation are called: Stoichiometric Coefficients (S.C.) 2H2 + O2 → 2H2O Balanced Here 2, 1, and 2 are stoichiometric coefficients. 9 Balancing Chemical Equations Hints for Balancing Chemical Equations: 1) Save single element molecules for last. 2) Try not to change the S.C. of a molecule containing an element that is already balanced. 3) If possible, begin with the most complex molecule that has no elements balanced. 10 Balancing Chemical Equations Hints for Balancing Chemical Equations: 4) Otherwise, trial and error!! 11 Balancing Chemical Equations CH4 + O2 → CO2 + H2O Example 1: Balance O2 last C is already balanced Start by changing S.C. of H2O to balance H CH4 + O2 → CO2 + 2H2O 12 Balancing Chemical Equations Example 1: CH4 + O2 → CO2 + 2H2O Now C and H are balanced Balance O by changing the S.C. of O2 CH4 + 2O2 → CO2 + 2H2O BALANCED! 13 Balancing Chemical Equations Example 2: B2H6 + O2 → B2O3 + H2O Balance O last B is already balanced Start by changing S.C. of H2O: B2H6 + O2 → B2O3 + 3H2O 14 Balancing Chemical Equations Example 2: B2H6 + O2 → B2O3 + 3H2O B and H are balanced Balance O by changing S.C. of O2 B2H6 + 3O2 → B2O3 + 3H2O BALANCED! 15