Dr. Chirie Sumanasekera Chapter 3: Stoichiometry 3.6.6 Percent composition (revision) 3.6.7 Empirical formula 3.6.8 Molecular formula AP Chemistry _ Notes Dr. Chirie Sumanasekera 10 /31/ 2017

3.6.6 Percent composition How much of each element or polyatomic ion is present in a compound on a percent basis. % of element = (atomic mass of element) (# of atoms in formula) x100 Molar mass of compound STEP-1: find grams of each In 100 g of compound : STEP-2: find % of each using the gram amount of each in 100g of compound:

3.6.6 Percent composition-problems Q1: what is the percentage of Fe, O, H and OH- ion in Fe(OH)3? Fe = 55.46 g O = 16 g Fe(OH)3 = 106.9 g Molar masses: OH- = 17.01 g H = 1.008 g Fe: O: H: OH- = 1:3:3:3 (a) (b) (c) (d) Molar ratios in 1 mol of Fe(OH)3 --> STEP-1: g of each In 100 g of Fe(OH)3: 1 mol Fe(OH)3 107.9 g Fe(OH)3 1 mol Fe 1 mol Fe(OH)3 55.46 g Fe 1 mol Fe (a) g Fe = 100g Fe(OH)3 = 52.2 g Fe (b) g O = 44.9g (c) g H = 2.8 g (d) g OH- = 47.7 g STEP-2: 100% of each In 100 g of Fe(OH)3: 55.46 g Fe 100g Fe(OH)3 (A) %Fe = x 100 = 52.2% Fe (B) % O = 44.9% (C) % H = 2.8 % (D) % OH- = 47.7 %

3.6.7 Empirical formula Simplest ratio of atoms in a compound. This formula is also used to represent ionic compounds (ex. H+Cl-, Na+Cl-) Molecular formula Empirical formula 1. Benzene C6H6 CH 2. Glucose C6H12O6 CH2O 3. Butyric acid HC4H7O2 (C4H8O2) C2H4O Calculations: Find the mole ratio of atoms in the compound Divide the number of moles of each element by the smallest mole value from 1. If amount of elements in the compound is given in grams, you need to convert grams into moles. If percentages of elements in a compound is given, then convert % in to grams….(if you are given only the mass of the compound, you can still find the mass of each element using molar mass of elements… Approximate decimal values to nearest whole numbers

3.6.8 Empirical formula Q1: what is the empirical formula of a compound that contains 4.0 g of calcium,7.1 g of Chlorine? Ca =40.08 g/mol Cl =35.45g/mol ? mol of Ca = 4g Ca 1 mol Ca 40.08 g Ca = 0.10 mol Ca ? mol of Cl = 7.1g Cl 1 mol Cl 35.45 g Cl = 0.20 mol Ca STEP-1:Find the mole ratio of atoms in the compound Empirical formula = CaCl2 1 mol Ca 0.10 0.2 mol Cl = 2 mol Cl STEP-2: Divide the number of moles of each element by the smallest mole value from 1

3.6.8 Molecular formula Could be the same as empirical formula or be a whole number multiple of empirical formula. Molar mass = smallest whole number Empirical formula Calculations: Find the empirical formula Molar mass of the compound is used to determine the molecular formula: Divide the molecular mass by empirical formula mass. Find the approximate whole number value. And multiply the empirical formula by that number Q1: A compound has an empirical formula CH2O and has a molar mass of 180g/mol. Find its molecular formula. empirical formula mass =12+ (1x2)+ 16 = 30g 180g/mol = 6 empirical formula units/mole 30g = 6 (CH2O) = C6H12O6