% Composition, Empirical Formulas, & Molecular Formulas Chapter 9 sections 3 & 4

% Composition % = (part / whole ) x 100 % = (part / whole ) x 100 When calculating the % composition, you are calculating the % of each element in a compound When calculating the % composition, you are calculating the % of each element in a compound

% Composition Calculate the % Composition of MgO Calculate the % Composition of MgO Mg = 24.31g O = 16.00g Total = 40.31g / X 100 = % Mg = 39.69% O

% Composition Calculate the % Composition of iron (III) oxide Calculate the % Composition of iron (III) oxide % Fe = 69.94% % Fe = 69.94% % O = 30.06% % O = 30.06%

Empirical & Molecular Formulas Empirical formula – the smallest whole number ratio of elements Empirical formula – the smallest whole number ratio of elements Molecular formula – the true number of elements in a compound Molecular formula – the true number of elements in a compound

Empirical Formula What is the empirical formula for H 2 O 2 ? What is the empirical formula for H 2 O 2 ? HO HO What is the empirical formula for C 6 H 12 O 6 ? What is the empirical formula for C 6 H 12 O 6 ? CH 2 O CH 2 O

Steps for Calculating the Empirical Formula 1. List your givens 2. Change % to grams 3. Change grams to moles 4. Divide everything by the smallest number of moles 5. Write your formula

Empirical Formula Problem Calculate the empirical formula of a compound containing % S and % O. Calculate the empirical formula of a compound containing % S and % O g S x g O x 1 mol S = g S 1 mol O = g O mol mol //// mol = mol = 3 SO 3

Empirical Formula Problem Calculate the empirical formula for a compound containing g C, 8.16 g H, and g O. Calculate the empirical formula for a compound containing g C, 8.16 g H, and g O. C 3 H 6 O 2 C 3 H 6 O 2

Steps for Calculating Molecular Formula 1. Calculate the empirical formula 2. Get the molecular mass of the empirical formula that you just determined 3. Divide the experimentally determined molecular mass (given) by the molecular mass of the empirical formula 4. You will get a whole number 5. Multiply everything in the empirical formula by this number

Molecular Formula Problem Calculate the molecular formula of a compound containing 40.68%C, 5.08%H, and 54.25%O with an experimentally determined molecular weight of g/mol Calculate the molecular formula of a compound containing 40.68%C, 5.08%H, and 54.25%O with an experimentally determined molecular weight of g/mol

Molecular Formula Problem 40.68g C x 1 mol C = / = (1)2 = g C g C 5.08g H x 1 mol H = 5.04 / = (1.5)2= g H 1.01 g H g Ox 1 mol O = / = (1)2 = g O g O C2H3O2C2H3O2C2H3O2C2H3O2

Molecular Formula Problem C 2 H 3 O 2 C 2 H 3 O 2 Molecular Mass = Molecular Mass = EDMM / EFMM = / = 2 EDMM / EFMM = / = 2 Molecular Formula Molecular Formula C 4 H 6 O 4 C 4 H 6 O 4

Molecular Formula Problem Calculate the molecular formula of a compound containing g C, 3.64 g H, and g O with an experimentally determined molecular mass of g/mol Calculate the molecular formula of a compound containing g C, 3.64 g H, and g O with an experimentally determined molecular mass of g/mol C 12 H 9 O 6 C 12 H 9 O 6