Stoichiometry Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice-Hall Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten

Stoichiometry Anatomy of a Chemical Equation CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

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 II. Balancing Equations

Stoichiometry A. Balancing Steps 1.Write the unbalanced equation. 2.Count atoms on each side. 3.Add coefficients to make #s equal. Coefficient  subscript = # of atoms 4.Reduce coefficients to lowest possible ratio, if necessary. 5.Double check atom balance!!!

Stoichiometry B. Helpful Tips Balance one element at a time. Update ALL atom counts after adding a coefficient. If an element appears more than once per side, balance it last. Balance polyatomic ions as single units. –“1 SO 4 ” instead of “1 S” and “4 O”

Stoichiometry C. Balancing Example Aluminum and copper(II) chloride react to form copper and aluminum chloride. Pg a, b, c

Stoichiometry Combination Reactions Examples: N 2 (g) + 3 H 2 (g)  2 NH 3 (g) C 3 H 6 (g) + Br 2 (l)  C 3 H 6 Br 2 (l) 2 Mg (s) + O 2 (g)  2 MgO (s) Two or more substances react to form one product

Stoichiometry 2 Mg (s) + O 2 (g)  2 MgO (s)

Stoichiometry Decomposition Reactions Examples: CaCO 3 (s)  CaO (s) + CO 2 (g) 2 KClO 3 (s)  2 KCl (s) + O 2 (g) 2 NaN 3 (s)  2 Na (s) + 3 N 2 (g) One substance breaks down into two or more substances

Stoichiometry Combustion Reactions Examples: CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O (g) Rapid reactions that produce a flame Most often involve hydrocarbons reacting with oxygen in the air Pg a, b, c, d

Stoichiometry Formula Weight (FW) Sum of the atomic weights for the atoms in a chemical formula So, the formula weight of calcium chloride, CaCl 2, would be Ca: 1(40.1 amu) + Cl: 2(35.5 amu) amu These are generally reported for ionic compounds

Stoichiometry Molecular Weight (MW) Sum of the atomic weights of the atoms in a molecule For the molecule ethane, C 2 H 6, the molecular weight would be C: 2(12.0 amu) + H: 6(1.0 amu) 30.0 amu Pg a, b, c, d

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 What is the purpose of % composition ? Determines quantity, How much of each ingredients

Stoichiometry A. Percentage Composition The percentage by mass of each element in a compound

Stoichiometry Percent Composition So the percentage of carbon in ethane is…C 2 H 6, %C = (2)(12.0 amu) (30.0 amu) 24.0 amu 30.0 amu = x 100 = 80.0% Pg a, b,

Stoichiometry Avogadro’s Number 6.02 x mole of 12 C has a mass of 12 g

Stoichiometry Molar Mass By definition, these are 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)

Stoichiometry Using Moles Moles provide a bridge from the molecular scale to the real-world scale

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 Practice problems Pg. 107 – (3.27, 3.33,)

Stoichiometry B. Empirical Formula C2H6C2H6 CH 3 reduce subscripts Smallest whole number ratio of atoms in a compound

Stoichiometry B. Purpose of Empirical Formula Tells you the substance of a formula and ratios

Stoichiometry Calculating Empirical Formulas One can calculate the empirical formula from the percent composition

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.

Stoichiometry Calculating Empirical Formulas Assuming g of para-aminobenzoic acid, C:61.31 g x = mol C H: 5.14 g x= 5.09 mol H N:10.21 g x= mol N O:23.33 g x = mol O 1 mol g 1 mol g 1 mol 1.01 g 1 mol g

Stoichiometry Calculating Empirical Formulas Calculate the mole ratio by dividing by the smallest number of moles: C:=  7 H:=  7 N:= O:=  mol mol 5.09 mol mol mol mol

Stoichiometry Calculating Empirical Formulas These are the subscripts for the empirical formula: C 7 H 7 NO 2 Practice Pg. 108 (3.43)

Stoichiometry Stoichiometric Calculations The coefficients in the balanced equation give the ratio of moles of reactants and products

Stoichiometry C. Molecular Formula “True Formula” - the actual number of atoms in a compound CH 3 C2H6C2H6 empirical formula molecular formula ?

Stoichiometry C. Molecular Formula 1. Find the empirical formula. 2. Find the empirical formula mass. 3. Divide the molecular mass (this has to be given to you) by the empirical mass. 4. Multiply each subscript by the answer from step 3.

Stoichiometry C. Molecular Formula The empirical formula for ethylene is CH 2. Find the molecular formula if the molecular mass is 28.1 g/mol? 28.1 g/mol g/mol = 2.00 empirical mass = g/mol (CH 2 ) 2  C 2 H 4 Practice Pg. 108 (3.51)