Quantitative Composition of Compounds Preparation for College Chemistry Luis Avila Columbia University Department of Chemistry

Atomic and Formula Masses Meaning of Atomic Masses Atomic Mass from Isotopic Composition Masses of Individual Atoms Formula Mass

Meaning of Atomic Masses Give relative masses of atoms based on C–12 scale. The Most common isotope of carbon is assigned an atomic mass of 12 amu. The amu is defined as 1/12 of the mass of one neutral carbon atom

Atomic Mass from Isotopic Composition

Atomic Mass from Isotopic Composition 20.00 (0.9092) + 21.00 (0.0026) 22.00 (0.0882) 20.18 amu A.M. Ne = 20.18g/mol

Masses of Individual Atoms The atomic masses of H, Cl, and Ni are H = 1.008 amu Cl = 35.45 amu Ni = 58.69 amu Therefore 1.008g H, 35.45g Cl, and 58.69g Ni all have the same number of atoms: NA NA = Avogadro’s number = 6.022 x 1023

Masses of Individual Atoms Mass of H atom: 1 H atom x = 1.674 x 10–24g Number of atoms in one gram of nickel: 1.00g Ni x = 1.026 x 1022 atoms 1.008g H 6.022 x 1023 atoms 6.022 x 1023 atoms Ni 58.69g Ni

Formula Mass The formula for water is H2O. What is its molar mass? 2H = 2(1.008g/mol) = 2.016 g/mol 1O = 1(16.00g/mol) = 16.00 g/mol 18.02 g/mol = molar mass of water

The Mole Meaning Molar Mass Mole - Mass Conversions

Meaning of Mole 1 mol = 6.022 x 1023 items Cl2 HCl H Cl 6.022 x 1023 molecules 6.022 x 1023 molecules 6.022 x 1023 atoms 6.022 x 1023 atoms 70.90 g Cl2 36.46g HCl 1.008g H 35.45g Cl 1 mol Cl2 1 mol HCl 1 at-gr H 1 at-gr H 1 molar mass Cl2 1 molar mass HCl 1 molar mass H 36.46g HCl

Molar Mass Generalizing from the previous examples, the molar mass, M, is numerically equal to the formula mass

Mole to Mass Conversion Calculate mass in grams of 13.2 mol CaCl2 110.98g CaCl2 1 mol CaCl2 mass = 13.2 mol CaCl2 x = 1.47 x 103g Calculate number of moles in 16.4g C6H12O6 1 mol C6H12O6 180.18g C6H12O6 moles = 16.4g C6H12O6 x = 9.10 x 10-2mol

Formulas % Composition from Formula % Composition from Experimental Data Empirical Formula from % Composition Molecular Formula from Empirical Formula

Mass % from Formula Percent composition of K2CrO7? molar mass K2CrO7 = (78.20 + 52.00 + 112.00)g/mol = 242.20g/mol 78.20 242.20 %K = x 100 = 32.29% 52.00 242.20 %Cr = x 100 = 21.47% 112.00 242.20 %O = x 100 = 46.24% Note that percents must add to 100

% Composition from Exp. Data Aluminum chloride is formed by reacting 13.43 g aluminum with 53.18 g chlorine What is the % composition of the compound? Calculate mass of compound formed Divide mass of each element by total mass of compound and multiply by 100.

Empirical Formula from % Empirical formula of compound containing 26.6% K, 35.4% Cr, 38.0% O work with 100g sample:26.6 g K, 35.4 g Cr, 38.0 g O moles K = 26.6g x = 0.680 mol K 1 mol 39.10g moles Cr = 35.4g x = 0.681 mol Cr 1 mol 52.00g

Empirical Formula from % moles O = 38.0g x = 2.38 mol O 1 mol 16.00g Note that 2.38 / 0.680 = 3.50 = 7 / 2 Empirical formula: K2Cr2O7 Potassium Dichromate

Empirical Formula from Analytical Data A sample of acetic acid (C, H, O atoms) weighing 1.000 g burns to give 1.446 g CO2 and 0.6001 g H2O. Empirical formula? Solution: find mass of C in sample (from CO2) find mass of H in sample (from H2O) find mass of O by difference

Empirical Formula from Analytical Data 2.02g H 18.02g H2O mass H = 0.6001g H2O x = 0.0673g H mass O = 1.00g – 0.394g – 0.067g = 0.539g O

Empirical Formula from Analytical Data 1 mol C 12.01g C moles C = 0.394g C x = 0.0328 mol C 1 mol H 1.008g H moles H = 0.0673g H x = 0.0668 mol H 1 mol O 16.00g O moles O = 0.533g O x = 0.0333 mol O Empirical formula is CH2O

Molecular Formula from Empirical Formula Must know molar mass Calculate empirical and molecular formulas of a compound that contains 80%C, 20%H, and has a molar mass of 30.00 g/mol.

Molecular Formula: (CH3 )2 = C2 H6 Divide each value by smaller number of moles Empirical Formula: CH3 Molecular Formula: (CH3 )2 = C2 H6