Chemical Quantities Empirical Formulas

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Empirical Formulas The empirical formula gives the lowest whole number ratio of the elements in a compound. The empirical formula gives the lowest whole number ratio of the elements in a compound. – Reduces the subscripts. The empirical is sometimes and sometimes not the same as the molecular or “actual” formula. The empirical is sometimes and sometimes not the same as the molecular or “actual” formula. – For ionic compounds, the formula unit is the empirical formula. Example when NOT: Example when NOT: – The empirical formula for H 2 O 2 is HO.

Empirical Formulas The molecular formula for vitamin C is C 6 H 8 O 6. What is the empirical formula? The molecular formula for vitamin C is C 6 H 8 O 6. What is the empirical formula? – A. CHO – B. C 3 H 4 O 3 – C. CH 2 O – D. C 2 H 4 O 2

Empirical Formulas Which of the following is an empirical formula? Which of the following is an empirical formula? – A. C 3 H 6 – B. NaCl – C. CH 4 – D. C 4 H 10

(Review): empirical formula tells relative number of atoms of each element One can calculate the empirical formula from the percent composition. The ratio of the number of moles of each element in a compound gives the subscripts in a compound’s empirical formula. Calculating Empirical Formulas “Percent to mass, mass to mol, divide by small, multiply ‘til whole!” 6

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. 7

Calculating Empirical Formulas Assume g of para-aminobenzoic acid, find mass of each element: C:61.31 % = g C H: 5.14 % = 5.14 g H N:10.21 % = g N O:23.33 % = g O 8

Calculating Empirical Formulas From mass, calculate the number of moles of each element: 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 9

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 Calculating Empirical Formulas Empirical formula: C 7 H 7 NO 2 10

Molecular Formula from Empirical Formula If we are given molar mass of the compound, we can determine the molecular formula The subscripts in the molecular formula of a substance are always a whole-number multiple of the corresponding subscripts in the empirical formula: whole number multiple = molar mass of actual compound molar mass of empirical formula Example: The empirical formula of a compound is found to be NaS 2 O 3. The molar mass of the compound is g/mol. Determine the molecular formula. molar mass of empirical formula = g/mol g/mol g/mol The molecular formula is Na 2 S 4 O 6. = Multiply each subscript in empirical formula by 2 to get subscripts of molecular formula

Example: Empirical and Molecular Formulas Ibuprofen, a headache remedy, contains 75.69% C, 8.80% H, and 15.51% O by mass. Its molar mass is 206 g/mol. Determine its empirical and molecular formulas. 12

Example: Empirical and Molecular Formulas Ibuprofen, a headache remedy, contains 75.69% C, 8.80% H, and 15.51% O by mass. Its molar mass is 206 g/mol. Determine its empirical and molecular formulas. 13 Empirical formula = C 13 H 18 O 2 Molecular formula = C 13 H 18 O 2

Chemical Quantities Empirical Formulas