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Percent Composition, Empirical and Molecular Formulas Courtesy www.lab-initio.com Calculating Percentage Composition Calculate the percentage composition of magnesium carbonate, MgCO3. Formula mass of magnesium carbonate: 24.31 g + 12.01 g + 3(16.00 g) = 84.32 g 24.31 Mg 100 28.83% 84.32 12.01 C 10014.24% 84.32 48.00 O 100 56.93% 84.32 100.00 Formulas Empirical formula: the lowest whole number ratio of atoms in a compound. Molecular formula: the true number of atoms of each element in the formula of a compound. molecular formula = (empirical formula)n molecular formula = C6H6 = (CH)6 empirical formula = CH Formulas (continued) Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio). Examples: NaCl Al2(SO4)3 MgCl2 K2CO3 Formulas (continued) Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio). Molecular: H2O C6H12O6 C12H22O11 Empirical: H2O CH2O C12H22O11 Empirical Formula Determination 1. Base calculation on 100 grams of compound. Determine moles of each element in 100 grams of compound. 2. Divide each value of moles by the smallest of the values. 3. Multiply each number by an integer to obtain all whole numbers. Empirical Formula Determination Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid? 1.Treat % as mass, and convert grams to moles 49.32 g carbon 1 m olcarbon 4.107m olcarbon 12.01g carbon 6.85 g hydrogen 1 m olhydrogen 6.78m olhydrogen 1.01g hydrogen 43.84 g oxygen 1 m oloxygen 2.74 m oloxygen 16.00 g oxygen Empirical Formula Determination 2. Divide each value of moles by the smallest of the values. Carbon: 4.107m olcarbon 1.50 2.74 m ol Hydrogen: 6.78m olhydrogen 2.74 m ol Oxygen: 2.47 2.74 m oloxygen 1.00 2.74 m ol Empirical Formula Determination 3. Multiply each number by an integer to obtain all whole numbers. Carbon: 1.50 x 2 3 Hydrogen: 2.50 x 2 5 Empirical formula: C3H5O2 Oxygen: 1.00 x 2 2 Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 1. Find the formula mass of C3H5O2 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g 2. Divide the molecular mass by the mass given by the emipirical formula. 146 2 73 Finding the Molecular Formula The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid? 146 2 73 (C3H5O2) x 2 = C6H10O4 3. Multiply the empirical formula by this number to get the molecular formula. Lets Practice Write the empirical formula for the following molecules. 1) C6H6 2) C8H18 3) WO2 4) C2H6O2 5) X39Y13 6) A compound with an empirical formula of C2OH4 and a molar mass of 88 grams per mole. What is the molecular formula of this compound? 7) A compound with an empirical formula of C4H4O and a molar mass of 136 grams per mole. What is the molecular formula of this compound? 8) An unknown compound was found to have a percent composition as follows: 47.0 % potassium, 14.5 % carbon, and 38.5 % oxygen. What is its empirical formula? If the true molar mass of the compound is 166.22 g/mol, what is its molecular formula? 9) Rubbing alcohol was found to contain 60.0 % carbon, 13.4 % hydrogen, and the remaining mass was due to oxygen. What is the empirical formula of rubbing alcohol? Answers 1) CH 2) C4H9 3) WO2 4) CH3O 5) X3Y 6) C4O2H8 7) C8H8O2 8) K2C2O4 9)C3H8O